USB 2 seems not to be a viable option on old world PowerMacs, as it is not on 68k Macs. But why would one want to use USB on an old machine? Flash memory is the reason, isn't it? Like digital photos, rugged and small removable media, compatibility with recent media to be shared with recent computers and peripherals. All those benefits are available, using recent flash memory with appropriate adapters. Think of a CF card or SD card from your digital camera, SDHC if you want. Any number of card readers and USB adapters is available for recent computers. And a few adapters for classic Macs are available, still. If you are interested in solutions, browse the 68kMLA Wiki.

A good point to start might be http://68kmla.org/wiki/Using_flash_memory.

Please do not forget to contribute results for your own setup on the page http://68kmla.org/wiki/Flash_Drive_Test_Results