Well, there are several issues at hand. First of all, you'd need a very high speed/high quality x86 emulator for PowerPC. Such a beast simply doesn't exist currently, at least not in the open source world, so that would have to be made first. Such an emulator would also have to include full MMU support, majorly slowing down memory accesses. This alone wouldn't exactly be a trivial project.

Next up, WINE itself expects to run on a Unix-ish system. That gives you 3 ways of moving forwards:
1) Try to modify WINE to rely less on Unix-ish systems. Not a trivial task.
2) Try to modify MorphOS to act more like a Unix-ish system, or create some layer that would do so. Not a trivial task either.
3) Skip WINE and reimplement Windows yourself. Obviously not a trivial task either.

So yeah, it's not that anyone hasn't thought about it, it's just that such a project would be a major software project, something nobody would have time for even if they had the skills to do it.