No ....haven't sailed a cat with a seat harness.
i have however,spent a lot of time in a dinghy trapeze.
When your sailing 'on the wire' you are usually parallel
with the water so you will have to support your upper body.
i recon i would last 2 mins with a seat harness.
I used a seat harness on a dinghy a fair bit in marginal conditions and found it comfortable but I agree that in fully trapping conditiond the full trapeze harness provides better support.
I've gone the other way and used a Hobie cat harness windsurfing (while windsurf commuting to the cat race.)
It was horrible.