The only difference in the USB Prober captures is the Device Class in the Device Descriptor.
I did some more research and found some interesting posts to the Apple developers lists about a very similar issue:
http://lists.apple.com/archives/usb/2010/May/msg00007.html
http://lists.apple.com/archives/usb/2010/May/msg00009.html
It's not exactly clear which version of OS X is being discussed, but it is probably < 10.6.8.
The posts seem to indicate there were limitations in the way OS X handled USB composite devices with CDC in older versions of OS X.
Based on this info, I don't think it would be worth the effort to try and pursue a solution that would work for all current versions of OS and also 10.6.8.
Thoughts?