I’m having the exact same problem; just wanted to post more information, hoping it’ll help someone answer this issue sooner.
When I run “Presto PageManager 4.0” (bundled with my Brother MFC 7820N) & select “File / Select Source / TWAIN…”, it brings up the TWAIN “Select Source” dialog and LISTS BOTH MY SCANNERS. The dialog looks IDENTICAL to the one TWAINClientCocoa opens, except that it lists my scanners…
When I run the “ImageCapture_Tiger_SDK”s “TWAIN_DS_Tester”, it also lists both my scanners. I can select one, and when I click “Start” all the handy TWAIN information about the scanner driver is listed just fine.
When I run TWAINClientCocoa on my G4 PowerBook, both my scanners are listed just fine.