Hrm, what if the mail box were adapted to serve this purpose...
Give it the house item appearance of an open book. Let vistors click on and use it. It sends a special mail message to the house owner, automagically filling in the owner's name (perhaps even the name of the person who placed it) invisibly. The vistor would just click, type a quick message about the home, and then send. If the decorator/owner wanted to, they could enter the message into a player written book to share with future vistors.
Of course, this might depend on a UI mod that allows the owner to copy recieved mails out of their inbox. I know my UI can do that, but I'm not sure about default.
At the end of the day, I'd just be happy with making mail boxes Vistor-usable.