Wear you climbing belt, put the fall arrest harness over top. When you are to climb out, put the lanyard around the tree (or climbing line) then remove the fall arrest. It's easier when you have the harness that clips rather than the one you have to climb into.
The reason the climbing belt isn't accepted as fall arrest is because you are connected at the front of the body. If you were to fall you would bend backwards at the waist causing back problems and I believe respiratory issues. A fall arrest harness supports you at the back so you would hang face down with the joints bending in a more natural position.
As I'm sure you know, a climbing belt is not fall arrest it is work positioning harness designed to catch a person who falls very little distance as there shouldn't be much slack in your climbing system.
I know you love to argue ad nauseum, but I'm not the one to argue with, the regs are the regs and I don't need the hassle or expense of a fine.
If you are doing this commercially, you should be following the safety regs, not making them up as you go.