As stated above, US Air/RAVN have modified the jets to include the radar, presumably all four now at Lakenheath, not sure how many of their others have been similarly modded.
As Mk67s were built with the 100 series longer nose but kept the 60 series internal fit/equipment they were well suited to the mod compared to the T1/50/60 series fleets.
With regard to the clearance, they are indeed on a US civil experimental registration, and accepted in the UK under reciprocal arrangements which makes a mockery of the number of hoops that the long term Design Organisation has to jump through to modify and fly UK aircraft.
After US Airs' two early accidents, which included one ground fatality, the USN/USMC blocked any further contracts. I'm not sure quite what the USAF's stance was at that time but after a company name change they are now using them without any Design Organisation support or approval of life extension past that when they were certified for the RoKAF.