it is meant to be a joke, and one based on Discworld at that, but it is very clearly a joke entry meant to let people know we're not trying to be NO FUN ALLOWED. If someone really did try to use "but that's a joke, so therefore all the rules are jokes" as an excuse, we'd just have to tell them otherwise.
In any case, joke entries in the Constitution have never been used as an excuse before to my knowledge, and it's been in existence for years*. I figure people who are able to comprehend the rules and follow them are also able to tell jokes from the serious entries.
*Older-bies, feel free to correct me if I'm uninformed. :P
Your suggestion looks pretty good to me, and looks like it includes everything mentioned so far, but there is one point that I'm not sure about, and that's J. I'm not aware of any official anti-mime... more