Maybe check around with some people who've done poop patrol (search the forum for the phrase to find them) and see if you could modify the idea as such:
Make self-adhesive easily-removable (think post-its!) stickers of a large scale to be applied to the interiors of the port-a-pots?
Another idea, less invasive and more participating first hand, is one that someone mentioned as a "wish" on another thread. Dress as G.W. with a mask and suit and then tape a "Kick Me" sign to your back! You'd probably get some affectionate paddling that would cheer up the paddler without hurting you. You do, however, run the risk of actually getting a running-start-drop-kick. It'd be a great way to meet people and participate, though!!!
"Sometimes, honey, you just have to look reality in the eye and say..... 'what happened to your other eye, dude?!'" - Angela Waversak, as quoted on eriswerks.org