...and player can't have by normal rules, true.

but, being as my player had a reasonable example of how this worked (Charmed, with Prudice and astral projection) I give him a flip coin option:

If he can design how it works, with fair side affects along with the benefits, I'll permit it.

Here's the rough idea so far.

mage projects, then manifests onto the material plane. Uses some form of a test (I'm thinking charisma or Conjuring) to manifest fully (as in corporeal) for a complex action. the number of successes you make on the roll determine how many turns (after the one you use to manifest) you can go before you start to suffer side effects.

time side affects so far:
stun drain equal to (half dice used)L. YOu roll to resist AFTER you go back to your physical body. Every time you go the number of turns again, the damage level increases. Once you hit deadly, you start to build up in physical damage (so stun at D and phys at L or higher is possible). If you roll resist in this case, you negate physical damage first, then stun. (so 5 successes against a 3D stun + 3M phys would only put you back to 3D stun).

other side effects.
spellcasting still does physical damage. Even though they have a body now, it's not their natural body.
You're considered dual-natured but you get the benefit of seeing the astral plane and material plane without penalty (the mental side effects don't affect your manifested body).
if you're killed in this form, you don't actually die, but you take two levels of physical damage (as per duration drain mentioned above). Likewise, you roll for magic loss

well, that's the list so far, it'll get update or whatever at some point.

Try this refugee from MitS:

Materailization Metamagic Technique

not bad, but I only have SR3 and Companion, nothing else.

Uhm, okay...

nah...this would get royally abused with my players so i wouldn't, but if you wanna allow it, maybe your players are more cool than mine.

You may want to simplify the rules too...seems like it could lag your game a bit, especially if it gets used a lot

If you don't have MiTS and therefore lack access to initiation and metamagic techniques, you might still consider using the rules for the metamagic technique linked to, as they seem in the spirit of what you want an a bit smoother. Just ignore the fact that it's a metamagic technique and the immunity to normal weapons bit.

This really seems powerful. If you allow it I would say that your corporeal form would manifest physical damage onto your body just as if it were really there receiving the wounds suffered by your corporeal form, and since you can't take your armor or weapons with you all you would have is your spell casting ability. I guess this would not prevent you from putting your armor and gear on after you manifest, but you would not be able to walk thru walls with it like a ghost. In fact you could limit it's usefulness by making the corporeal form appear nude as an exact copy of your physical body, but with absolutely no gear, which clothing is a part of. Also a minimum of +2 to all target numbers to represent the lack of physical control over corporeal functions with your mind. In a sense you are operating a puppet with your mind that has a special link to your meat body. I would also suggest your physical attributes are based on your astral attributes, but maybe with a big penalty due to the restriction of physical space with gravity and mundane means of movement. Half strength astral attributes for corporeal form attributes? You are controlling this with your mind, from afar, but with the handicap of real world physics. Possibly reduce perception as well? Would you be able to feel? Taste? Or are you just limited to sight and sound like astral projection?

There is alot to consider. Players could astrally project into any lab in the world. Look thru a computer sabotage data, or steal it. They could easily by pass alot of stuff if they could turn off alarms by manifesting inside of buildings for example where previously characters could have projected to see what was there, but they couldn't get in without setting them off etc.. They could destroy whole buildings by sabotaging gas lines letting the basement fill up with gas, then maybe rigging up a phone to a source of ignition, blink back to their meat body, make a call, and boom, goodbye skyscraper. The potential for abuse is huge. Probably nothing that can't be done by an ally spirit manifesting in human form, but at least the ally has to be payed for with alot of karma.

Is this a meta magic technique you are exploring? I think it would add a whole new meaning to the words "go f*#k yourself"? Don't you agree?

If you can consistantly make 8 successes using nothing but Willpower every 3 seconds against a TN that grows each time to deal with the Physical Deadly Drain it causes, you deserve to be able to "abuse" these rules.

Since few magicians will doubtfully have a Willpower higher than 6, that's not going to happen. Only a single die of those 6 dice have to come up as a 1 on the very first Combat Turn to give them a Moderate Physical Wound that can't be healed with magic. And it just gets worse each turn thereafter. They'll likely be dead and disrupted within 10 seconds.