I've been thinking maybe I could use a condom, you know pee in it and then toss it... It seems a little uncomfotable in theory, but with practice... I found this was a pretty big problem at the NYC Tri as you are on the West Side Highway, there is practically no place to even go in the woods, just stone walls all the way, and then I couldn't find the john at the end. When I did, what a relief.
I like the suggestion of pee while biking and then washing yourself off afterwards. That makes sense. Maybe more than a condom.