I think they've kind of written themselves into a corner with the PC being able to be a mage, to be honest. It's like if they let you play a version of Baldur's Gate where 2/3 of the classes WEREN'T Bhaalspawn. I think there are several ways to handle the handwaving that results from the attempt to treat classes that are not socially equal as if they were socially equal:
1. Don't let the PC be a mage dammit. (not recommended--mages are my favorite class and everyone knows that I get to have everything I want, right?)
2. MAKE the PC be a mage. Dammit. (I wouldn't mind, myself, but the screaming from all quarters would probably get on my nerves.)
3. Have the mage have a completely different experience based on being a mage. (Unfair unless they basically institute class-based origins and have them majorly affect the game, which would mean essentially writing THREE COMPLETELY SEPARATE GAMES and gluing them together somehow. Needless to say, while this would be the coolest option, it is also the most resource-intensive and least likely to happen.
4. Cheese, i.e. everyone is miraculously unaware that Hawke is a mage EVEN WHEN HAWKE POINTS THIS OUT. While the least restrictive and resource-intensive option, this also can look REALLY STUPID in game and lead to all kinds of wonky immersion-breaking paradoxes.
5. The option I'd (tentatively) recommend even though it's a HUGE cop-out: make your mage PC be some kind of special 1 in 10,000 mage who is allergic to blood magic and makes demons break out in hives (or vice versa). Of course, over several games, this cop out would look increasingly goofy (are you telling me that I'm playing EVERY SINGLE Safety Mage in the WORLD?!) but it'd at least be internally consistent without making the writers tear out their hair.
6. Invent a time machine, go back in time, and smack whoever came up with the idea of making mages radically socially different than the other playable classes without subsequently coming up with a way to implement this regarding the PC without, again, MAKING THE WRITERS TEAR OUT THEIR HAIR.
7. You could also just make Warrior and Rogue PC social pariahs/outcasts for different reasons, and just write the entire game as everyone you ever talk to freaking out about you being a. Kossith (that'd be your social pariah warrior) b.) an elf or a dwarf everyone hates for some reason (your rogue) or c.) a mage. Actually, you could do this even more simply by letting any race play any appropriate class-except that humans *have* to be mages. EVERYBODY'S an underdog!
Hmm, actually I think I may switch my recommendation to #7 there. That'd be (in my mind) a great way to combine classes and races AND different origins without resulting in a crapload more work or a wildly inconsistent/restrictive game. You could even call it: "Dragon Age: Outcasts". And I suppose if you wanted to you could come up with a way that humans could be warriors/rogues and still fit in with the outcast theme.
Actually, I really like that idea.
Sorry for the long rambling post, I was making it up as I went along.