I made a similar experience using a cable modem through USB, which is a similar case. I searched the web as thorough as possible regarding cable modems and support. I also tried DSL modems and USB-ethernet converters. I found not a single one which is supported by MacOS. All of them were Windows98 only. I also tried the USB Development Kit, where I found a Enetshimlib for USB and similar stuff. No succes with my modem, though. Anyone any luck with those beasts using a USB-ethernet adapter?
My last try is to push Apple.