I agree, that many kite harnesses could be a bit oversized. But on the other hand if you start riding realy well powered and want to go upwind for some km, you may come to the conclusion that a slim climbing belt could become unconfortable, at least my experience from time I was riding lowpowered but hooked with kite.
For winging I actually use an 20 year old neilpryde waist harness, still in perfect shape, and don't see too much reason to search for anything else. To me it works also well switched without any slider.
The only additionaly specific demand for winging to me could be to have a good connector for the board strap, that is independend from the hook, for the case that this brakes, to not additionally lose the board.
And if you are looking for something inbetween climbing belt and windsurfing you may find quite some ultra light harnesses, as for example this which I think could be interesting for winging and low powered mostly unhooked kitefoiling: