If it's for blocking signs, I'd say the best bet is around that one area, put everybody into gamemode 3, and then when they actually go somewhere away from the signs, gamemode [whatever gamemode we on]. But this idea is pretty good, I support.
There is a command in vanilla servers for minecraft that allows players to be semi transparent allowing them to basically be ghosts. This would take out the need of plugins and it still allows players to have full play functionality they just don't have to fully see a player.