I use the Black Dog Head Harness… mainly because it doesn't slip upwards on the nose, and into the eyes. The way it's designed, it stays PUT (and even VERY loose) over the nose. When I introduced it, I used it in conjunction with very high value treats (as you're doing). My dog only got the reward after he was in a sit-wait and the harness was clipped on though. Instead of using the reward treats as bait (to get your dog out from hiding), try to teach the dog to do a sit-wait, maintain the wait, clip the harness on, then release, reward, and go for your walk (the walk is also a reward).
it's much like teaching your dog to sit still for other things such as nail clipping, ear cleaning, teeth brushing, hooking the leash up to the regular neck collar, etc... I use the sit & wait while I do those, and I do the same when putting on the head harness.