I don't know if it's possible, but I do believe that some items can be tagged so as not to be taken by the clean-up scripts. I suggest that custom notes written by player characters to represent posters and other messages be immune to the clean-up script. As it stands now, except in the Mistlocke outdoor map, they're quickly gobbled up and removed from where they were set.
I've always felt that these notes added to the color and roleplay of EfU, but right now, unless you drop them in the square, they're unlikely to be around for anyone to see.