I don't want to seem to quibble of trivialities, but this is getting me crazy.
1) This does not constitute evidence of any weight toward the accusation.
2) He possibly could have hidden this message, but before we go assigning any kind of probability, let's look at what was actually drawn.
3) The moon in the sky is used to indicate night. I think of using the outhouse at night as just a well established meme, and it's probably connected to the tradition of the crescent moon on the door. I'll repeat what I said above: Islam doesn't have a monopoly on crescent moons. Jeez, next someone will accuse the French of stealing the idea of the croissant from Algeria.
4) I've already explained about the SLAM. Many comic strip artists, and particularly Hart, use the same overall image for all of the panels in the strip, in this case the outhouse with the door shut. I assume this is for the convenience of not having to draw much to produce a new strip. But whatever the reason, he will use other techniques to convey action, such as "action" text between panels. This is the easiest way for him to convey that the character seen in the first panel has entered the outhouse. Why do you burden Hart with the responsibility of doing the strip a different way, just so the audience can't cook up an imagined insult, in a way he probably couldn't have thought of in advance?
So to your opening statement, I still see zero reason to believe that Hart is guilty. That's a lot less chance than "not the most likely."