Thanks for the tip. I picked up a used usb protocol analyzer for a good price to diagnose the immediate problem (lots of NAKs at an unexpected time) but I'd still like to pursue getting USB support into wine and not have to depend on a Windows box (or VM) for any part of the project.
Fair enough. Well I happen to know Peter Stuge who is the guy behind libusb (http://www.libusb.org/). He tends to hang out at #sigrok under the username "CareBear" .
I would recommend having a chat with him.
Joel