Have you ever tried one of those goofy seats with no nose? I tried one once that was just two round bits: one for each sitz bone. (then I got a Terry Sport and the world was a beautiful place)
It's a bit harder to control the bike without a nose on the seat, but it might be worth trying one out at the LBS just to see if it'll work for you.
I assume you've already tried the diaper wipe thing.
Have you had a bike fit by a PT?
I'm gonna PM you with another suggestion that's not ummmm well, just not what you want to see posted on a public board...
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson