Just wanted to post the progress I made today, in case this might help someone else.
I compiled “TWAINClientCocoa” on my PPC-based Mac, and copied the resulting .app to my Intel-based Mac. When I fired it up, the TWAIN “Select Source” dialog listed both of my scanners.
So (as a work-around) if you build your TWAIN-based application on a PPC-based Mac, it will be able to properly render the “Select Scanner” dialog with your system’s list of scanners!