There’s a good scene in “Truth in 24″ (the documentary following the Audi team at Le Mans). Alan McNish says to Tom Christensen something along the lines of “The seat is a little wet, do you do a little pee-pee?”
If the need arises, the drivers just go.
However, as part of the preparations before a race, a driver will empty themselves of waste products. That gives me an excuse to post this video. http://youtu.be/ILmODpW92r8
The drivers do lose a lot of water from sweat, so they generally don’t need to go. However, they do take a lot of fluids on to combat dehydration, so it can happen.
They wear several layers of quite thick clothing, which absorbs quite a lot of fluid.