Possible, I'd say yes. Practical, I'd say no.
I'd never heard of the Riored thing, but that's basically what you'd have to do. As current busses to northbridge are much faster than G4, there may be timing problems if there aren't wait states in the bus protocol. The northbridge needs to be able to waste time while the PPC/FPGA provide data, and it also needs to be able to stop sending data to CPU when it can talk much faster than the FPGA/CPU can listen. But I'd guess it's still "possible".
Then you'd need a BIOS on the CPU adaptor card for the PPC. I don't think you'd really need a new BIS for the motherboard, but an x86 emulation would be handy to "run" whatever is native to the motherboard. An extension to the graphics BIOS emulation already in uboot...
Even if it could be made to function, it'd still be akward to fit into the AMD or Intel sockets of today and then fit a heatsink properly. Could be tricky. But not "impossible".
All in all, to get enough knowledge of modern northbridge busses and get the logic translation working in an FPGA would be a lot of work. It'd probably be easier with one of the coming SOCs that have DDR and PCI-Express built-in, or maybe a G5+northbridge all on the CPU adaptor card. Then you could either hypertransport out of the card for the G5+northbridge, or finangle a hypertransport/PCI-Express bridge to that part of the AMD CPU socket, then connect the DDR interface directly to that part of the AMD CPU socket, etc. AMD sockets might be easier to fit into as hypertransport is documented, and so is DDR, I don't know what else is in an AMD socket these days. I don't know what Intel's protocol is or if it's documented in a form useful to these crazy ideas. I'd rather do G5+northbridge than try to inangle an FPGA to translate directly from G5 elastic bus to Intel bus.
But, using a G5+northbridge that way, is kindof akward sounding, plugging an almost complete computer into another computer. I don't know of a PCI-Express to Hypertranport bridge that works in that direction, the ones I have heard of between those two need the CPU connected to hypertransport end and cannot handle CPU conencted to PCI-Express end, which is exactly backwards to being able to use it with an 8641 or PA Semi. I'm not against plugging a nearly complete computer into another computer, but some don't like the idea.
I think the amont of work involved and the expense make this very unlikely, but I wouldnt' call it "impossible".
