Ok, did a little more investigating. Seems like some sort of gremlin more than anything.
So, with the Keyboard and mouse attached directly to the G4 (Apple Pro Keyboard and Mouse, OEM for this machine), I start up, and the ports are unusable on the display. None show up in the System Profiler except the built-in ports. Then, if I disconnect the display from the card, and reconnect it, suddenly the ports on the display are active, show up in the System Profiler, and work with the keyboard/mouse. However, if I then restart the Mac, the ports are inactive yet again. Unlpugging and replugging the monitor with the keyboard still plugged into the monitor does NOT reactivate the ports. Only if the keyboard is not plugged in when unplugging/replugging the monitor do the ports come alive again. Strange, hmm? I don't. know how to explain it.