how to regulate use of USB keys through group membership

Sorry for the rude shorthand but i'am a bit rushed

We have a vWorkspace 7.2 MR1 windows 2008 r2 RDSH farm.

Our users have HP thin clients that are not domain members

We want to control use of USB keys through the use of AD groups

We can only allow disk drives in the local resources of the pit file on the thin clients but since these things are not in AD we can't use groups

We tried masking with the NoDrives regkey in HKCU but the USB key has no drive letter. It just shows up in explorer

There is a GPO that regulates this but only a machine policy. No use on a terminal server...

Any ideas?