...except when it's not.
There's no apparent joke there on the surface, so I wouldn't go so far as to say that the message is secret; it's not obscured by anything else. Hart's message is just composed of an artful arrangement of visual symbolism that supplies the context that gives meaning to words that are otherwise a non sequiter.
The first thing I noticed was the six crescent moons, especially the ones on the outhouse door. That Hart means that to signify the "house of Islam" is hardly subtle. I didn't even catch the "SLAM" until I read SteveS's message.
You're trying pretty hard to come up with reasons why the obvious message isn't the message, but I notice you haven't supplied the joke that Hart "really" meant. Care to fill us in?