I think most people wouldn't pay much attention to it all. Might be worth a punt but I'm not convinced.
I think people outside the WT couldn't care less, which means, the billboards will be catering specifically people who have interest in the WT, who may or may not pass by them and read it for a few seconds.
And then the message written has to speak to them, and it's a well known fact that many people join the WT (and everything else for that matter) join for many different reasons. What exactly will it have to say? Additionally, there are many JWs who can't even articulate some of their teachings, leave alone use what they read outside about it. So the billboard may have to:
- Be located where/when a specific group of people can see it
- Catch their attention
- Be about something they understand or care to learn more from an outside source
- Be about something that speaks to their own specific situation
Doesn't sound to me like a venture worthy of time and money.