It was already expected that support for PowerMac G4 machines would arrive - in fact, it's the one reason why I keep my own PowerMac G4 dual 450Mhz around. We already did a little digging ourselves, with little results. Hopefully, this means I can write an addendum to our MorphOS review shortly.
Apparantly, it should technically be the case that all PowerMac G4 models are supported, but it will most likely come down to which chipsets and components were used by Apple for each individual model. One constant: you'll need an ATI Radeon video card, as NVIDIA chips are not supported by MorphOS - at all. We'll need to wait until the actual release of a new MorphOS version with this support built-in ("sooner rather than later") before we can get an accurate and detailed overview of which models are supported.
This is great news, and support for PowerMac G4 machines is much more useful to aspiring MorphOS users than Mac mini support. For some inexplicable reason, G4 Mac mini machines are still quite expensive, while PowerMac G4 models are cheaper than toilet paper. Do make sure, however, that any G4 PowerMac you buy for MorphOS has an ATI Radeon card.