25+ years ago when I was at Eglin shooting AMRAAMs from Dunsfold's finest wonder-jet, the MU-2 was used as a range clearance aircraft and weapon TM re-rad, especially when shooting down low and well out to sea.
As a related aside, great photos of XZ439 from Vzlet on the "Royal Navy F-35B compared with anything" thread - good to see it is still going strong.