The graffiti appears to be part of Hulu's marketing campaign for "The Handmaid's Tale," their upcoming television series inspired by Margaret Atwood's novel about a dystopian society where the female gender has been completely oppressed and fertile women are treated like objects. The text is a secret subversive phrase spread throughout the novel that translates, in psuedo-Latin, to "Don't let the bastards grind you down."
"The Handmaid's Tale" is still currently in production for a 2017 release date, so the show doesn't have much of a presence at the convention beyond these cryptic bathroom signs. But make no mistake, this is a brilliant way to spread the show's message directly to its target audience — and in a space that, despite the best efforts of geeky women and activists alike, is still considered very male-dominated.
Don't let the bastards grind you down, indeed.
Check out the "In Production" teaser that Hulu put out this weekend: