Modder's Zone closed off unless RB2 modding is figured out.
I have a feeling people will look into this as PS3/X360 emulating improves.

Currently possible:
Swap Skillset - YES [chaospunishment has done this]
Extract Sounds - YES
Extract Models - YES
Swap Models - YES
Swap Sounds - UNTESTED
Replace Sounds - UNTESTED
Replace Models - NO
New Content - NO

Local files related to the character model are in the following formats:

.spr
.ioram
.vram