dimarie might have meant the captains seats- the front ones in newer vans and I suppose many cars too - that can recline.
The reclining position might help take some pressure off the fanny ?

you might have looked thru these already- it's hard to find the right search terms- I mostly got wheelchair cushions - but one of those might work for how you would use it also.
Cushion relieves back, buttock pain and sitting fatigue

http://www.wheelchairparts.com/pad.html
http://www.rehabmart.com/category/Wh...r_Cushions.htm