Kernel 2.4.19-r5 of ppc-sources boot with a blank screen on PowerBook DVI withATI Mobility Radeon 7500. Mac OS X and 9 are installed on that machine too. I just compile the kernel without changing any configurations. There is no problems booting with 2.4.19-r4 though.
What can you tell me about 2.4.19-r6?
Also you may need to tweak conf config options in the kernel for your machine
2.4.19-r6 has been around for a while; give it a shot.