This sort of worked, in that the next bootloader would start, and that could load the kernel and initrd, but then farther along in the boot USB I/O would fail, grinding everything to a halt.
I eventually figured out the USB stick I was using was too new, and it was handling some USB 1.0 state differently than the USB controller was expecting.
Switched to an ancient USB stick I found lying around and things worked!
The BIOS has an allowlist of wireless cards, which the AR922X is much too new. I found a firmware with the check disabled, but the author notes the BIOS still has the radio disable pin set.
Fortunately the AR922X seems to ignore this pin, as the radio works.
@mavica_again I was thinking something along the lines of Qmodem Pro (or the modern OS clone, Qmodem)