After following up on a couple of tips here, and doing some browsing on the web, I think I'm heading toward a Toyota Landcruiser with a full-length roof-rack. Some of the roof-racks shown online have floors made of bars that are tight enough to stand on without any further flooring, or that you could just throw a piece of carpet on. The bars could also be used to secure both myself and a tripod. I currently have a Lexus GX470, which is a version of the Toyota 4Runner, and it has been very reliable (though I have busted both a tire and a windshield, not the vehicle's fault.)
I did find, in browsing, a "photographers" vehicle here:www.sportsmobile.com/3_photographers.html
That's a little more than I want. I also live in Minnesota, where the winters are tough, and I really want to be able to keep the truck in my own garage.
I did some browsing on Land Rovers, which somebody suggested, and though the LR has great after-market stuff, like roof-racks, there are two problems in the U.S. The first is that the Land Rovers have a bad record for reliability, and are expensive to fix when they do break down; and the second is that most states have only a single LR dealer. That means that if you break down in West Cooter, Arkansas, you might have to truck it a couple of hundred miles to get it fixed. The most recent Land Rovers have a high-performance Jaguar engine; they're supposed to be great for off-road, but I think that would be most practical in off-road L.A. Toyota dealerships,, on the other hand, are everywhere.
I thought about something removable that I could just put on, and take off, the roof. That was my first idea, in fact. But the problem is, most SUV roofs, including mine, are actually curved in several directions, and in differing degrees. If I just threw a few pieces of plywood on top (perhaps joined with hinges, with rubber on the bottom to protect the roof) I'm afraid that I'd still be denting the high spots under the flat plywood...and I think trying to match the curve of the roof with plywood would be impossible. The other thing is, anything removable would probably just get buried under all the other crap I drag around with me, and discourage impulse-shooting, which I don't want to do. I'd like something where I can yank the car off the road and be set up two minutes later.
I'm thinking that if I attach something like a bungee cord to the bottom of the tripod's center column, and then to one of the roof-rack's floor bars, it should be really steady. I plan to tie myself in with safety lines used in sailing; a two-point hitch, one on each side of the truck, would at least keep me from falling off and landing on my head.
If I actually pull this off (maybe this summer?) I'll post a picture.