This is odd and i might add "old" i had the same behavior when i first purchased the powerbook but it was fixed, then i was booting from the local drive.
Yes the screen is full of scrolling garbage and the colors look halfway right (greenish on white, later black on white).
I netbooted using these configurations:
i) 6.99.23 distribution (kernel plus userland, generic kernel -thus radeonfb is compiled in)
ii) 6.99.15 kernel and 6.99.23 userland. The kernel is the one i am using to boot
from the hard disk. When local booting everything is fine but not when used for netboot. It has radeonfb support with extra info (debug plus additional pci info)
iii) 6.99.23 distribution (kernel plus userland) with radeonfb disabled as you requested.
All the configs had the same behavior.
Openfirmware info.
Commands used:
dev screen
.properties
display
type: 4c434400
name: ATY, Jasper_a
compatible: ATY, Jasper
address: bc08000
there is plenty of hex output, let me know if you need anything more (if the device i selected is the correct one).