If you can still find the DVD, "Road to Paris", there is a section in there about race pace and what goes on during a stage, including relieving oneself...and a nice shot of a USPS rider going (from the back) from the bike with another pushing him along.

There are bunches of photos of whole lines of riders all along the roadside taking care of #1. It's when #2comes along, and in a grand tour it can get nasty with an almost daily liquid diet, that things get dicey. All kinds of stories about riders getting the runs, going, and teammates washing him off with waterbottles.

The race stops for no one...