Nothing is impossible, but I'm going to go with "Not gonna happen" in a big way.
The fans aren't the biggest issue--at least in theory those could just run full blast, as they would if you booted into any situation without fan control, like Linux or FW disk mode.
The problem is OS9 doesn't have drivers to support any of the other hardware on the G5s, either. OS9 has no built-in USB2 support that I know of, it definitely has no SATA support (didn't even have ATA controller support for the late model G4s), it has no FFW800 support, it probably can't do anything with the optical audio I/O, it has no support for whatever new chipset the motherboard might use (which I'd guess is significantly different from the previous G4s, due to PCI-X, etc), and if the G5s themselves need any special coding to be used by the OS, it certainly doesn't have that, either.
Not that it'd be impossible to add any of these things, but Apple's certainly not going to do it, and I don't think anybody else has the inside knowledge that would be required to do it behind their backs.
Bottom line being this is more than just a little tweaking to support "unsupported" hardware, there's a big difference between the G5s and the last models OS9 supported. That's a gap I don't see being bridged.