I use my windsurfing harness all the time. In fact, I got a full catamaran harness the end of last year and have yet to use it.

That said, my harness (DaKine Slalom Seat) offers very little back support. It is a very low harness. That's why I ended up getting the full harness, so that while racing, hopefully it will be easier to hike fully flat. The windsurfing harness is nice because it's very simple and the spreader bar (which all cat harnesses have now anyway) makes it comfortable.

My advice, try before you buy if it's possible.

sm