i'm in your conscience
BW wouldn't work either. tbh, that's too similar to a normal run, cause a lot of people use Pokemon introduced in that region when playing normally. Something like using Pokemon not introduced in that region would work though, I think. So say I went through Hoenn with only gen II Pokemon...I could end up with Crobat, Donphan, Lanturn, Magcargo, Skarmory, and Bellossom. ...Not a bad team actually...now I kind want this WHOOPS haha.
But yeah, that would shorten the options so you'd have to think about your team. You could also have an optional rule that doesn't allow you to use items that were made past that gen. So if I were doing the challenge I mentioned in my example, I would only be able to use items that were around in gen I and II, not anything introduced in gen III. This actually sounds like a good challenge lol...sorry I hijacked your idea by turning it around and making something new of it. ~_~