There are PowerPC emulators out there. I believe QEMU emulates the full G4 instruction set (can't recall if it supports G5 or not,) but it doesn't emulate any OS4 custom hardware
What OS4 custom hardware? Taking my A1 as an example, apart from the CPU, nothing much else I can see is custom. It's all made out of standard PC parts! I don't see the problem here. UBoot is downloadable and ACiube even offer the source on their server!
I'm surprised no one has made an A1 emulator yet. Compared to the complex original Amiga, which has been emulated for years, an A1 is easy! You got no custom chips to think about. Either those coders are lazy, or they think it's too hard and don't bother. Hey, my A1 can emulate a Mac in Linux, so there's no reason why a PC couldn't emulate an A1.