I like the idea. It shouldn't be to terribly difficult to integrate some sort of system like this. A lot of games have found ways to implement similar ideas from gear color matching to pulling stats off better gear and slapping it on lower level patterns.
The most likely way for SE to execute this based on what i've read (and not having peaked at their coding) would be to tie it to the new doll suit (forget SE's name for it) that they already have in play. Since each additional one of these gear dolls has to be be won/bought anyway it should be simple to add an extra "costume" doll for each class that is unlockable. Then it would work very similar to what the OP described. The Top doll would have your stat gear, the second or under doll would have the "Costume" gear and determine how you look.
I doubt this would be something able to be added by launch unless the code already exists but is dummied out. But unless there is a major design conflict I could see them adding this in the not to distant (post 2.0) future.
Just my two cents on implementation.