Wow, way too complicated for me. :-(
I wish I could help. Here are some issues:
(1) This would be a big mod, because it would require that you download a copy of the graphics files for that monk.
(2) I do not know how IWD2 determines what avatar goes with what class. I would have to understand that control structure in order to change it.
(3) There is no guarantee (although it is highly likely) that the frame sequences and whatnot will line up -- it's possible that the BG2 one won't work unmodified.
If you can find anyone who can explain how IWD2 picks the avatars (and note that they seem to be based on your highest class level) based on 2DA files or something, let me know.
Much like, say, effective character level penalties or racial favored classes, it could be hard-coded into the EXE.