It looks like a good idea, i willl try to keep any of my my extra characters as copy-Paste as i possibly can so they can be shared in your archive.
Also, it might be a good idea to keep a section for special entities, like "Bosser", timer, endlevel, warp, etc
you would have to ask permission from some of the coders - possibly. they can be found here in the forums, but to have something like Mugen archive for OpneBOr seems OK