The armor visuals will be the biggest hurdle, but maybe you can reuse existing armor models and just slightly resize them to your model (or just make your creation the same size as existing ones.)
Yep, if you could successfully import your mesh you could replace pretty much anything or add new characters/npcs/statics, it's just that the entire mesh import pipeline was so undocumented and unintuitive, starting from the GR2 format to some of the unwritten rules you had to obey for animations (not no mention having to swap normal map channels ...), that it was pretty hard to make things work. It didn't help that the editor doesn't really tell you what's wrong with your mesh, it just won't display properly.
