Funnily enough, part of the kit issued to Combat Engineer Tractor Operators was a large oval rubber bladder with a steel cup at the top and a valve at the bottom. In the case of operating in an NBC (or ABC for our european comrades) environment it was used as a pee bottle. The fact that you got to share it with your crewmate was a bit horrible though! I never needed to use one though, as I didn't take training to such extremes!
http://www.uribag.com/uribag.php ?

Was stuck in hospital a month ago, and rigged up to a pump and monitors for a few days, so had to bottle it. 5AM is a bad time to find out your bladder can hold more than the bottles they give you, is all I will say.