RAF AWACS controllers have regularly controlled UAV's (RQ-1/MQ-9 Predators, RQ-4 Global Hawks primarily) for many years over various trouble spots and war zones. Many of these areas have had extremely high traffic densities, and sometimes most other aircraft have been non-squawking for operational reasons. Essentially, controlling a UAV is no different to controlling a manned aircraft: there's still a pilot answering your calls, it's just that he happens not to be co-located with the callsign!
Regards,
M2