I was able to get a libusb-1.0 async version of this control message working once I "stumbled" on the fact that the payload portion of the message was 8 bytes long. Who knew ?
I have been able to "distill" a whack of detailed protocol information about the Phidgets 8-8-8 Interface Kit from the existing C codebase. However, this is far from the most efficient way to go about this exercise.
Meanwhile all my code and progress in this adventure is documented (and freely available) at:http://www.icanprogram.com/opndrs/phidgets
If anyone wants to jump in and help with the code polishing/documenting/extending ... all volunteers are welcome. It is an open source project.