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.
JR