I cannot possibly speak authoritatively, but it seems that the crux of your problem and the possibility is your statement that the pilot is requesting the heading and has visual contact with the ground (and therefore is maintaining visual separation from terrain)
ATC is therefore NOT vectoring. You are not telling the pilot to fly a certain heading, he is requesting a heading. I would expect, if making the same sort of request, to hear something like this-----"Cleared to avoid at your discretion, maintain visual separation from the ground, inform me when you can proceed on course."
All over the world, I have had similar clearances. Take a MSA.....It relates to a safe altitude, based on an obstacle within 25 miles of the (for example) VOR. So this could be a hill, a radio mast or what-ever. If the pilot knows this obstacle, and it could be a tower he has passed already or a hilltop 24 nm away, then he knows he is safe, even though he is lower than the MSA, because he knows his position relative to the obstacle or terrain feature.
Approaching a ridge south of Albuequeque, ATC got an automated terrain warning on me. I told him I had good VMC, terrain in sight and I would remain at present altitude and maintain own separation from terrain. Roger. I was landing at an uncontrolled airport on the other side of the ridge and climbing to his MVA for the area was impractical and unnecessary.
Hope that helps