That's exactly the setup I have and what I was thinking of doing.
A bit more detail, I was pondering this for a while, but kind of had the idea gel when I freed up a 7-port hub I had been using. It has 5 vertical ports on the front and two on the rear along with the B connector and power connector. It's a little narrower than a 3.5" drive, but about the same height as a 3.5" hard drive.
My thought was to somehow hard mount it to a 5.25" adapter (floppy/zip bracket) and put it in the lower bay. Except for a cable to power the hub from the optical bay PSU connection, the rest looks like cake. I'm just wondering if I'm missing any gotchas on the bay doors, etc.
13" MBP (late 2010), MDD G4/1000x2, Sawtooth G4/800, Pismo G3/500, Lombard G3/333, B&W G4/400