I doubt it. There's too many ways for this not to work - the police testifies "I never saw that billboard". Even if they admit they saw it, so what? Anybody can say anything on a billboard, the police officer could say "Oh yeah, I saw it, but I have no idea if it's true" - and plausible deniability is sufficient as I understand it.
You can even put it on their qualification tests so you have documentation that they heard something was illegal and it doesn't matter. For many things they are effectively immune to prosecution. Add on that most prosecutors don't want to go after cops because they won't cooperate in cases after that.
