There's a person on the Garden that uploaded a system folder containing the 9.2.2 installation from the restore disc from my machine, and it doesn't include any extensions that look relevant. So I don't think it's drivers or anything like that.
Having spent some more time with the machine...
OK, so it is possible! Do you have anything special in extensions or control panel, beyond Energy Saver? Nothing like a Powerbook Extras extension or something?
I bought myself a very nice 867 MHz Titanium Powerbook to run OS 9. It came with Leopard installed and I used it for a short while before I swapped the spinning drive for an SSD and installed OS 9 by mounting it as a Firewire Target Disk and copying the OS 9.2.2 installation over from my G4...
TBH even just having labelled solder pads would be fine, an (optional) 2.54mm header would also work.
I don't think that's a stupid idea at all. An internal port could also be used for USB wifi or bluetooth/wireless keyboard connectivity.
One extra external USB port probably isn't so interesting, no. I use my Lombard for making music, and once I sketched out an audio-over-USB/MIDI-over-serial interface that would fit in the battery bay (with a flex cable to the modem connector on the mainboard), but to call that niche is probably...
The Powerbook Lombard (and I suspect Pismo) use the same modem board. I put a Stealth Serial port in mine. It doesn't fit physically, unfortunately, but I did get it to work with the keyboard detached.
Allegedly there's an extra USB port on the connector (see the pinout Alex Hixon published)...
I get that, but I'm wondering if it's more work to figure out how they did than to just make a new driver that acts as a serial port. Assuming that you'd be making new hardware anyway.
Isn't ADB completely documented? Would it be substantially less work to do that from nothing? I assume you want to reuse their drivers, but are they really that much work?
Have you tried following the traces backwards from the jacks? It'll go through at least some passives, maybe the opamps. Either it goes through the custom IC or directly to the port.
I'm counting 5 quad op amps (LF347) in the half of the board closest to the output, so I strongly doubt it's sending all-digital signals to the Patch Panel. There's also the Yamaha YMF chip, which is a synth IC, and it's companion DAC, the YAC512, so there is definitely something analogue...