Raspberry Pi port, piCore-15.0 is available!
Clarification might be needed. Are you trying to access a local device (which is not on the remote machine)? If so, are you sure this is a feature of rdesktop?
Are you sure? No mention of that in the rdesktop man page. Google says the -r usb is a feature of the virtualbox fork of rdesktop, not the original.
OK, what is the issue?