After reading more, even for wearing them under running shorts I'm coming around to bibs. I have the same concern about going over the fence to take leak. These Pearl Izumi Elite bibs, which seem to be pretty decent and popular, seem pretty tall to pull down far enough. I suppose worst come worst one could use a bathroom stall and take off the straps.

http://www.amazon.com/Pearl-Izumi-El...ct_top?ie=UTF8