The issue with harness-style belts is that they restrict movement - that's what they're designed to do. For regular driving and general trail riding, that's actually quite inconvenient - can't lean forward to reach the CD, reach back for a bottle of water, etc. Similar to when your automatic belts lock because the truck is on a really steep incline (btdt, and it's quite a nuisance...). So what I see the guys with harnesses do on the trail is they're not using the shoulder belts but only lock the lap belts. I understand why - to keep mobility in the shoulder area - but it doesn't make much sense to me to go thru the trouble of installing a harness if you're not using it. Just counterintuitive and all that.
One solution might be to use a V-style harness with a roller/automatic section attached at the end of a long 'V'. I can't remember who makes(made?) these; mounting point was either behind the back seat, or they were clipped into the lap belt on the back bench.
I originally was sceptical about having a seat w/o reclining adjustability, but after several trips halfway across the country and a whole bunch of trails in the Mastercraft Pro, I have no issues or regrets.