to gain what?
Only force dice as a bonus to their unarmed combat skill, right?
Killing hands, critical strike and the other stuff in combination are usually so much better than anything a weapon focus can do . .
Especially seeing how, with distance strike, they can do stuff the weapon focus can't do.

Additional Dice to add to all those Already Cool Adept Powers, of course, Since they WOULD be additive...

On the downside, the focus will be astrally active by that time. So passing through wards will be quite difficult, even more if said adept cannot see the ward. An enchanted ring would serve the same purpose, I think. It comes down to whether hardliner gloves and stuff like that stack on killing hands and such.
I, after all, don't have that much of a problem with implanted foci. One of my characters stored a power focus in a hidden compartment in his lower cyberarm. My GM outright told me that if the right crowd ever found out that my character had a diamond worth 50.000 in there, he'd meet a buzz saw very quickly. That was way before I found out that the arm itself could have been enchanted as well...
As Stahlseele pointed out, tattoos are much more suspect to damage than implants. They are also more noticeable. It's always much easier searching for a guy with a fully tattooed hand, he will likely stand out without a disguise. No disguising himself as a CEO, either - tattoos will be rather uncommon at that pay grade.
If nothing else, I even think you could enchant the bones themselves, if you manage to take them out and put them back in afterwards.
Which rules, in particular, forbid a weapon focus for unarmed combat? I've even heard of an adept with enchanted chopsticks, from my point of view you can use pretty much everything as a weapon.
For further inspiration, some X-Men character named Ink who had that idea quite before.

Assuming you mean your own bones, that isn`t possible. And cyberwear is only enchantable before it is put on. Living objects cannot be made into foci (so, no, you can`t have a snake power focus coild around your neck), and neither can anything paid for with essance.

And I don`t think there is anything that says one can`t have a weapon focus for unarmed. It just doesn`t come up that often since the only weapon listed in the books that works with it is the Hardliner gloves, and unarmed is generally not very effective in combat, unless you caught with your pants down, so to speak.

Yes, Uncle Drake thought of it first.
And yes, in canon, Frosty has an enchanted hip bone i think . .

Other weapons for "unarmed" use come to mind. Brass knuckles, kubotan, lots of rings... those aren't covered by the rules. Hardliner gloves just give a certain precedent, I'd say. Otherwise those weapons would require an "Exotic weapon" skill.
I got the rules in another way: No part of your body can be enchanted, because they are bound to your aura (well, Stahlseele's discoveries aside. I like the cage, by the way.). If you take them out, they no longer do. Otherwise used ware would be prohibited, too - it has at a time been part of a body, anyway. The way I see it, only living things are excluded. Animal's bones are useable material for foci, too.
To get back to the topic, I still think tattoos would work.

Basically, they'd work because we'd say they would. Either they just get a pass, or they get a special handwavy metamagic or something; otherwise, we're down to arguing that the dye particles are the focus, or something. And that's all fine, as far as I'm concerned. The only way a tattoo focus would differ mechanically from implant (non-cyber) foci would be unarmed weapon foci, so that's just something to be *aware* of. You can decide for yourself if you want to make that (pretty minor) change in your game world.

Oh, I just found a disadvantage of this "unarmed" weapon focus: It may be restricted to one hand. Unarmed combat is, as far as I recall, generally the use of both hands, let alone legs or elbows. Precedent for this is that if one uses two (cyber)arms with different strengh and agility, the average is taken. I don't agree with the idea of a focus consisting of two completely independently moving parts respectively fists. Just hitting with one arm might force a malus from the GM. I'd issue a -2 for that, with the possibily to be countered by a specialisation of unarmed combat. I'm also on house rule territory by now.
By now I get an idea why enchanted hardliner gloves are so tricky within the rules.

Possibly, but the rules don't really care about hands. You can 'lead attacks' with different cyberlimbs, and the basic rules don't include penalties for whatever number of limbs you use (1, or more). You could certainly house rule it, though.

What about nanotattoos on the inside of your body?

But unarmed combat could be just punching with one hand. For instance, if you have a shock hand, you attack with unarmed combat and deal damage according to the shock hand.

The Jopp, I think the book mentions that: you'd have to enchant each nanite individually, and they get destroyed after like a week regardless.

Plus, AFAIK the weapon focus has to be the thing hitting the target; anything under the skin wouldn't work. Obviously, this detail is what the 'weapon tattoo focus' house rule would change, though.

Ah, right, i forgot about nanotatts having a half life time measured in days . .
Thank you for reminding me of this.
As for enchanting bones: Etcher Nanites. Now you don't even need to flay yourself open to inscribe your still living skeleton with runes for enchantment . .
And Bone-Lacing . . who ever said using orichalcum for this does not work? O.o

You could probably do that, but it'd be any kind of focus except a weapon… unless you make a small house rule saying it works. Otherwise, a weapon focus is an object that you hit someone with directly.

hmm . . Well . . your knuckles could probably survive without that bit of skin over them . .
"OK, that's enough, now the gloves come off!"

And that'd be badass. Walking around with shiny (bone) knuckles cuz the skin's all torn off, jesus.

"Been punching people for so long i wore out the skin over my knuckles . . decided 'hey, why not use it as an advantage?' And now i have shiny mystical metal spirit slaying knuckles that make me punch out dragons if i want to!"

For the record i'd allow it, but i'd also hit someone with a small essence hit or other penalty, you introducing another magical aura into your own afterall.

It's your game of course, but i can't see there being and basis in fluff nor crunch for this O.o
Technically, awakening is introducing a magical aura into your own, so . .

And normal Foci don't cost you Essence, even when they are integrated into your Aura, as they mostly are when you wear them against your skin.

I don't think the extra few dice would really break the game. But I could be wrong.
Given that if an Adept wanted this to boost his magical fists, and magicians already have enough answers to spirits, I doubt tattoo weapon focus helps much at all.

I guess the thinking for me is if introducing cyberware, even relatively non invasive cyberware into your system affects your essence ritually changing a portion of your body to introduce a permanent focus should as well.

Prepping a vessel for possession is a ritual that changes your entire body/spirit but doesn't affect essence at all. I'm sure there are plenty of other examples of ritually changing your body/aura (initiation?) that don't negatively impact your essence.

Ah, now I get what you were talking about. Well, took me a few days, my bad
A branding then, perhaps? Though it might prove difficult to stuff any focus materials there.
Anyway, I wouldn't forbid the use of a sheathed sword weapon focus either... damage value might be another point there, though. It is most likely up to GM fiat.