Is there a Defect (or even a Quality) that will cause a Magician to manifest a highly visible Shamanic mask any time they're casting or sustaining? Basically, I'm designing a Magical Girl character for a one shot and need something to represent her transformation when she's using Magic. I've thought about using Signature or Distinctive Style, but neither of those quite fits. Suggestions?

There's no quality. It's simply a player's choice whether or not it'll happen for a shamanistic magician whenever they use magical skills (SR4a, 181). I presume this is intended to be an all or nothing kind of deal for life, not one that a shamanistic magician can choose whenever they decide to cast a spell.

You could have it as a Geas that the shamanic mask is clearly visible while casting or sustaining spells.

Ah, that's cool then. I'll just have to let him know then. Also, is it possible to get an Ally Spirit at start? Do I have to use Karmagen to get it, or is it only available after start?

That I'd say is up to the GM, and would certainly be easier to do in karmagen than BP. IIRC this requires an Initiation and the metamagic feat for summoning an ally spirit, typically not something a starting character can have.

I missed that somehow. Well, that solves that problem. Thanks!

A shamanic mask is, unfortunately, one of those really nifty/cool things that for some reason is only mentioned for the Shaman tradition.

I would second the Geas as being appropriate, because it's the kind of thing that should be part of other shaman-like traditions, not just specifically Shamanism(the core book flavor)

Its worth noting that an active mask can confer some social bonuses/penalties - its mentioned offhand in like, reference to Intimidation, I think, either in street magic or the core book. It will also make it pretty much impossible to cast subtly.

Shamanic mask is actually mentioned as an option for Bear Sarks (berserkers) following the Norse tradition in Street Magic (pg 39) as well as for spirits possessing vessels (pg 95), so I'd say it would be reasonable to assume that other Shamanic-style traditions would also posses the option of having one.

I can't find a reference to it enabling social bonuses or penalties though. The only thing I can find on it really is that it makes Perception tests easier (-2 threshold) when trying to spot someone using magic (SR4A 179).

With that said, I'd agree that it should be considered part of a Geas, similarly to chanting or ritual motions that also make spotting magic use easier.

I'll see if I can dig it up. I remember it being exceptionally obscure, and possibly related to situational social/appearance modifiers, like that from biosculpting or being excessively augmented.

The Geas defect could work, but I've already got a Talisman Geas because I need my (as yet undecided) magical thingy to help me "transform." Two Geasa seems a bit much and I don't wanna eat up all of my defect points just being a Magical Girl. I had considered making it a possession tradition to get the mask from that (like Mr Shed mentioned), but decided against it because of all the inherent unbalance possession traditions bring to games. Well, that and it's a crapton of paperwork to keep track of that stuff.

I've already taken Moon Maiden as a Mentor Spirit and I'm seriously considering giving her a minor Delusion about actually being a reincarnated Sailor Scout, so obviously I'm not taking it too seriously.

Just check it with your GM, as there's nothing saying that non-Shamanic traditions can't have a Shamanic mask, especially since there doesn't seem to be any benefit to doing it and you have a Mentor Spirit who's mask you could manifest.

Looked through Street Magic, and it notes in there that Shamanic adepts "... have been known to exhibit shamanic mask when calling on their powers." (page 45) Personally I can't see anything solid to deny this to other traditions, especially if they're following an animal mentor spirit and/or are of a naturalist kind of tradition.

Didn't find anything about social benefits/penalties, though.

Well, you could say that a hermetic mage could follow artificier mentor spirit and gain the bonuses but displays arcane/scientific floating holoschematics when he casts spells or tinker with magical items.

Its all in the description.