Hang on, just had a thought.

The question mark has to be a good sign, right? It means a classic ROM is operative on the system.

When I tried using my eMac install disks on the FW800, I got a System 9.2.2 folder choice in the Startup Disk options, but when I actually restarted that way, I didn't get a question mark. It just hung and then restarted in X.