It's simple if you're a man.....
That reminds me, I've been wanting to invent a way to pee while driving without having to stop. It takes a good bit of time to pull off the interstate, park, go inside and pee, and then get back up to speed.
Depends, really.
TapaTalkin'
I wear this thing while riding that's basically like a depends. It's hard to explain, but bikers know. Basically just a butt pad in a short kind of thing. I've put that on when it hadn't quite dried fully because I had to leave for a ride and even the slight wetness of that was disconcerting.
I think like a little funnel that comes out from under the seat, with a hose that routes to underneath the car would be cool. Just make sure no one is behind you.