You can trick OSX to be installed on any PC if you have the hacks to patch the installer. It isn't straight forward though and doesn't work with every PC.

As for running Amiga OS4 on a PPC Mac. Some have supposedly achieved it on a Mac Mini. Here is a video showing it in action. There also seem to be quite a few torrent hits for the Mac Mini hacked version of Amiga OS 4. Not sure about compatibility with other PPC macs though.