You're asking about the efficacy of passive messaging - I don't think it would work if you asked people. Likewise, I think simply doing it is more likely to annoy people than incite passion.
Visibility is the issue, not understanding the problem. I think the message of "the internet could be slower without net neutrality" is the one that needs to be delivered. People generally know what "slower" means.
As I mentioned, a standardized message would probably be more effective, but it's also not something that would be particularly exciting to share on, say, Hacker News.
I daresay annoying people is the whole point. Combine that with the logo example given upthread, and you've got a very effective campaign going on. What I see happening:
"Why do I keep getting this bloody message with Comcast's logo on it every time I hit random websites? readreadread Fucking Comcast at it again.."
Our impression (and experience) is that people get more excited about doing something symbolic.
Again, any direct recommendations about how to improve this, and we'll improve it :)