Are you saying to mount the boom high enough that it swings over the HP boom?
I don't think it's really matters if it's above or below the HP boom. I've never seen anybody use the dryer boom from wall to wall. I'd say max is about 135° past it's home position. Most customers figure out the extra hose quickly but there is a small group out there that could get tangled up in a ball of yarn...
A few more things:
If needed, remember you can invert the boom to lower the location of the fasteners and raise the boom. Yeah, it looks a bit messier but if it solves a problem....
Second thing is to take note of how much the inlet on the boom swings, if I remember correctly, it was about 6 or 8" so don't get too close with your PVC or you'll create an issue where the flex hose doesn't allow for the desired movement.
I tried Vern's approach as well and it solved a problem on one of my installations. It took about an inch of shim at the base to get a result.
Putting a small stainless steel washer under the bottom of the boom between the boom and the mount (and then drop the bolt through) so the boom rides on the washer instead of the whole mount seemed to help with reducing the resistance too.