Does the onsen have an outdoor pond with a communal bath area?
Do you know what the towel situation is going to be?
I was thinking maybe you can just forget your towel somewhere and just go around bare, but I'm pretty sure the locals won't mind since it's mandatory to go in naked after all.
Maybe 'accidentally' slip and fall with your leg spread so you expose your buttcrack?
Be very careful not to hurt yourself, though.