I live in orienteering country, but I'm an ordinary runner. If I'm caught short I look out for one of those orange and white flags that orienteers leave out specially for people who need to visit the "rest room". It's really thoughtful of them.
I think someone comes round every now and then and collects it all up. At least, that's what the guy waving seemed to be doing last time I used one of their "depots" but I'm not sure as I had ipod earbuds in and couldn't hear him.