I'm getting into this discussion a little late, so bear with me.
I just purchased the same set up, and while following the directions, noticed the same issue. I did a Google search, and while going blind on videos, noticed one set up where the waist belt was threaded through the two slots just above the bottom of the back-plate. It was not Hollis by the way, but a good look showed it would work, so this is what I did:
I cut out the grommet on the waist belt, and melted the ends. Then I threaded the belt through the two slots (just above the last mounting hole on the bottom). I started from the back, went through the farthest slot (nearest center line), then back through the outside slot. I left about 6" of webbing, then threaded on one of Hollis's metal slot buckles. I then tightened that up, and worked the waist belt webbing back through the slots so the buckle prevented the waist belt webbing from pulling through.
I then attached the hardware to the waist belt webbing.
Not knowing if this would hold, I put the plate on the ground, stood on it, and pulled (I'm 6'2", 205), and it didn't budge.
I figured this would be worth a shot, and at worst I would be out a waist belt.
Hope this helps