That, IMO, would be asking for a lot of trouble that you really don't need, in order to supply a "feature" that virtually all sensible users really don't want.

Frankly, the world is just not ready yet for passwords containing non-ASCII characters. Not only do we lack stable/consistent conventions for their keyboard entry, but their encoding and transmission are still mired in an embarrassingly wide range of "alternative methods" and obscure constraints. Forget about it.