It's been a few years since I've done any work with netinstall, but I remember back in the Tiger days, it was necessary to have two separate netinstall images to boot the new Intel machines and the older PPC machines. (Tiger wasn't universal; it was one or the other.) Leopard is supposedly universal, but if you're having trouble, it might be worthwhile to keep two separate images, one built from an Intel machine and one built from a PPC machine.
More specifically, what goes wrong when you try to boot from PPC? Do the machines not find the image? Do they TFTP the kernel and crash when they try to start booting? Or are you having problems with the netinstall/netrestore procedure itself once the machines are booted?