True, there are more and more ECV's out there with tires you can't let any air out (or in). Has it's pros and it's cons. Even if it is possible, you'ld only want to do it if you can put more pressure back in afterwards. While less pressure reduces pain caused by bumps, it does make the moving around heavier. Not something you'll want in parks that are smoother.
Freemore, if ever the day has to come that the car can't be fixed anymore, look at the chairs in it for comfort. They can do loads these days to reduces bumps by ajusting the chair, both as a standard option that comes with the car and as a true aid. Most of them are relatively affordable. If that isn't enough, think about those airpressure systems they've got for the drivingseat of a truck. My powerchair has such a system especially designed for wheelchairs. I know there are products out here for chairs in cars also. I don't know if it's out there in the US yet, but for someone with a proper mechanical background they're pretty easy to build.
If you ever have to use/look into using a manual chair; there are those out there with extra bump-reducing 'thingies' (sorry, don't know the word in english) in the frame and cambridge. Also now there are wheels -both the big and caster wheels- that have a bump-reducer built in in the center.