Here's what worked. Thank you for everyone who helped - one of the subtleties did seem to be booting all the way on the GPU. In a few months when I've more time for it I will start bisecting my way towards a more modern kernel, but regardless I'm going to stick with Debian for now.
This is one of those journeys where I have ended up back where I started but with much greater knowledge of an unfamiliar system.
Hardware configuration:
* VGA disable jumper applied.
* WX9100 connected to monitor via MiniDP -> HDMI.
Petitboot (etc.) configuration:
* Fast reset *not* disabled.
* Boot console: /dev/hvc0.
* (Current interface: /dev/tty0)
Host OS configuration:
* Debian 11.3 installed via hvc0 then switched to Linux kernel 5.5 from RaptorCS git compiled for POWER9 and 4k page sizes.
* Boot image rebuilt to ensure amdgpu module included.
* Kernel args added: modprobe.blacklist=ast video=offb:off console=tty0
* xorg.conf.d configuration snippet to set amdgpu rather than modesetting as the module to use for amdgpu:
https://wiki.raptorcs.com/wiki/Troubleshooting/GPU#Alternative_Xorg_configuration_using_OutputClass_and_PrimaryGPU* MATE desktop environment.
How to boot:
* Boot with GPU output connected to monitor. The fan will spin up, and the screen will go blue or black at various points in the process. When the keyboard state LEDs flash it's about to reach PetitBoot.
* If left alone it will autoboot all the way through to the desktop, although the signal will drop with a blue video screen briefly after kexec and the host OS kernel amdgpu module will show some alarming-looking errors (ring test failed, KCQ enable failed, etc.).
* Log in to your desktop environment via the session manager and enjoy!