Bill this may not be what you're really looking for, but there's a company down here in Oz called AirHawk that make air seats for bikes cars trucks heavy machinery etc.
Here's a link:
http://www.airhawk.com.au/AIRHAWK-Cu...-Cushions.html
I have one on my motorcycle and the difference it made to my comfort over 800 - 900 klm days was astounding. The trick is to inflate it just enough so that your behind is just clear, like about 1/8th inch, of the seat. Too much air turns it into a trampoline. On the bike its other advantage is that it allows lovely cooling air to circulate around your err crown jewels which further adds to the comfort. Not sure that you'd receive this added benefit in a car -- you might...
A friend of mine has mounted his AirHawk under the seat cover of his bike so that it's not visible but the seat looks a bit lumpy now.
I haven't tried one of these in a car but if they work as well as mine does on the bike, your problem is solved. You'll just have to pop it in the trunk when parked.