If it was to enter service I think you would see the "MQ" designator used since it would be Multi-role - conducting both Surveillance and Attack roles along possibly with EW and other roles.
In fact, NG are supposedly offering a derivative of their X-47 for the USN Unmanned Carrier-Launched Surveillance and Strike (UCLASS) program. This is to develop an aircraft carrier-based unmanned aerial vehicle to provide an unmanned intelligence and strike asset to the fleet. The UCLASS will be "an autonomous aircraft capable of precision strike in a contested environment, and it is expected to grow and expand its missions so that it is capable of extended range intelligence, surveillance and reconnaissance, electronic warfare, tanking, and maritime domain awareness" - therefore the MQ designator would make sense.