It's just a thought. I like giving my players options about these sorts of things. Perhaps three lists: THe 'no way in hell' list (banned completely), the 'not in your dreams' list (black list that the feats use) and the 'okay you can have it' list (th white list). That said, I specialize in feats, races and classes, so if you need help with those, I should be able to do it a bit easier than the legends thing. (Oh right, the legends...)
I've posted a decent deal about allomancy in the Magic thread on my forum, as well as a link to the wiki with the original concept. I'm still working on it, since it's a conversion, but I hope that you'll like what you see.