Been reflecting on all of this for several days. Bottom line I'm still not putting the 4-point rollbar in my car immediately. However, in the meta-analysis, while the points about the relative safety of a 4-point rollbar in a mixed-used car are valid, everything's a tradeoff in the end.
Most likely if I decided to take this car off the road before doing the 4-point, I'd do so by buying a cheap used truck for DD to haul parts/tools/wheels around in, but still be driving the car itself to track events. Then the next step would be buying a trailer the truck can haul the car around on. Honestly, any cheap used truck I buy is going to be way down the safety scale from the 370 regardless of whether the 370 happened to have a 4-point rollbar in it.
So really, I'm not saving myself any real safety on the street by putting off the 4-point. I'm still going to buy the truck anyways and drive it anyways, knowing that it's less-safe. It's all relative.
However, having read up more on all the issues with seat-back bracing, sliders, FIA seats, blah blah... I'm inclined now to wait to do the 4-point until I first fix up the seating situation. I'll probably sell the recliners off and pick up some fixed FIA-certified buckets and mount them solidly to the floor, and then do the 4-point with the harness bar at whatever the right height ends up being for my shoulders in those seats.
That and use some of the street-friendlier foam on the bar, and I should be good to go for a reasonable risk tradeoff. There's a couple of SFI-compliant high density foam options which have a softer outer part to help out with the helmetless bit. It's not perfect, but nothing is. e.g.
BSCI - Roll Bar Padding | Energy Impact Systems