I like the billboard idea. Some mantras and images should be kept secret, but from what I understand, those that are specifically designated "liberation through hearing/seeing" can be shared publically, e.g. song of the vajra. Someone correct me if I'm wrong ASAP please!
The only thing to be careful about is not to broadcast/display in such a way that people will have a negative reaction, which may not create a good cause. Although, maybe any contact would be positive.