LCD protest panel
I wonder if this is a good idea:
A light-weight LCD screen, like an e-reader, which flashes a message for what ... 1/3rd of a second or whatever a blipvert was.
It could cycle through a range of blank screens, inane messages (like 'this sign is intentionally blank') or whatever as well as the real messages.
You could even have a panic button to wipe the memory of whatever the 'naughty' message might be.
Vive la quiet and restrained revolution, where we all get to go home for tea and biscuits afterwards.
mines the one with 'I'm a conformist' written on the back.