Don't forget that some also use multiple characters in one post. Particularly advanced-level GMs. We'd think having a [char=#] [/char] Mcode would work. That way, you can have multiple characters in use in a single post. This Mcode could have the paramater # so that you can have 2-3 x hexadecimal characters of characters defined. 

Having a character tab in each RP, you would not have to look far to find that RP's characters. One way to make that extra advanced would be for any post you make in that, to be linked to a tab in your own profile, where you can browse any and all character sheet posts you've made. (i.e. posts made in Character sheet tabs). Edit one, and the other listing of it is also changed.

Interchangeable avatars/sigs could also be interesting. It might be confusing, but it could bear investigation. Most would still probably stick to just one or two, though.
Profiles should have something visible that doesn't change for such, to make it easier for people to "recognize" others. Be it avatar, sig or something else. (not just name, as not all are good at remembering such.)