iMac update: It's alive!
The second image is how the machine looks with LT-Enet bridging set up and running. The MacIPPi is connected over ethernet, the PowerBook is connected to the serial port.
Everything is working perfectly on the OS 9 side of things but I had some trouble booting OS X with the card installed. I will investigate that and report back with what I find.
I am going to need some assistance getting a breakout board designed for the prior model of iMac G4. I have an EMC 1987 here and would specifically like to see the modem breakout from EMC 1873 as I believe it is quite different to the 1987 version.
There are only three early-access boards left now!