The chance of coincidence between the 'abc' in the URI and an original 'abc' is much greater in that case, but not at all guaranteed. Well, maybe, except that it not only gives a 400, it also confuses or kills any other part of the Web infrastructure that handles URIs. Just a bit too much to risk. Ok, now it's clear what your problem is. It would have helped to know it up front.
Forms are indeed a serious problem, but there is some help, see below.
So if you use UTF-8 and label your page as such, it should just work.