It would be borderline insanity for a F 111 or any other to be flying 500k at ground level in traffic. If they are going to do that, it should be in one of the many military restricted areas than can extend from ground to 18,000 feet. Anyone who designs one of these low level routes to go through an area where there is a lot of other traffic at low level would be foolish.
I don't think military planes flying at high speeds have anything to do with this topic of hang glider flying. I flew hang gliders in 3 locations in California in the middle of military bases and never saw a military plane anywhere near.
If you were in command of a B-52 and the plane and crew were lost because you ran over a hang glider at low level and high speed Ithink it would be major strike against one.