It's never been an issue for me, but since most of my riding is done in a rural, fields and woods environment, I wouldn't have an issue with taking a squat in the woods if the need arose. The TP thing is a bit more problematic, but since I usually wear a bandana around my head for a sweatband, I guess I'd sacrifice it if necessary. That is, if there weren't any old corn cobs laying around.

My digestive system generally shuts down when I'm exercising, which is probably why it's never been a problem for me. Keep the breaks short, and I'm good to go (or, technically speaking not go).