Since I work in construction, the porta-loos are always covered by peoples attempts at wit. Most of them revolve around:
"So-and-So like some unnatural sexual act!"
"So-and-So is an asskisser/apple-polisher/bootlicker/etc."
"Free (insert trade name here) certificates!" with an arrow pointing to the crap-wrapper container.
The best I can remember, though, was some back-and-forth discussion about some particular guy on a site, between himself and some other tradesman, and it had gotten to an exchange of fighting words by the time I saw it; I chipped in with "Why don't you just step inside and settle this like women?: and signed my name to it.
Funny enough, non one ever asked my about it, although I overhead people discussing it for a couple days afterwards.