you can take extra clothes, just like ce-ce said, and most venues have medical areas, where they treat people for minor things, like a bad fall or something. My husband is a nurse, and when he works special events team, he gets to go to all kinds of things, that is why i know that!
if you took a dr note, like was suggested earlier, and showed it at the medical office, they would most likely let you use that room, and then, not only would it be private, you would be with nurses or emt's and they would be understanding. Also, don't worry about the smell, i have been in bathrooms during concerts, lol.
i go all kinds of places and am gone sometimes all day, and just empty it if i have to. dont go without eating complelty though, or you will get gas! what i like to do, is change the day before i go to work, that way if i am going to get a leak, i will already know. But you shuold be alot more comfy with everything by then. and since he does already know, you won;t have to worry about that!
i say, GO AND HAVE FUN!