Hi @ Aminah,
Yeah, I know what you mean about sarcasm (including “the trickiness of moderating around that”), and in fact I almost decided not to mention it because of this. I like your term ‘good-natured’ which is what I meant about some kinds of sarcasm not being a problematic, also that certain kinds of ‘good-natured sarcastic’ humor can at times, actually help prevent or diffuse a difficult situation. In addition, personally I wouldn’t want this type of guideline to cause people to ‘worry’ about every little thing they post; that could have a negative effect of overly inhibiting the natural flow of a fruitful discussion. Well, I guess I’m making a case here for just leaving any mention of sarcasm out 
But regarding putting some mention of it in, the only thing I can think of at the moment would be to refer to it as an example in relation to something else. For example, under passive-aggresive: “One example of this kind of behavior could be seen in back-handed sarcasm through the use of condescending language (or ‘a condescending tone’)”, or saying something meant sarcastically, e.g. “thanks a lot”, when you actually mean quite the opposite.” I realize even something like this veers into difficult territory, judgement about a person’s intentions…
Overall, I think the best approach is to use quotes from the Buddha (as already being done and as suggested further) to support the excellent points already made in the guidelines. In the end we can only each look into our own mind and honestly reflect on where our intentions and responses are coming from. And we can still be deluded:-) I love your reminder of the teaching in MN104. Also, yes MN103 is the sutta I was thinking of–thanks. I find it very clarifying and powerful.
As for your last question, hmmm, not sure about a specific formulation…the more I think about it, the more I think it may already be sufficiently covered, implicitly, in the guidelines. If it seems helpful to add, perhaps just a simple ‘reminder’ statement near the end of the more specific points: such as: “Remember, our own behavior, as reflected in the tone of our posts, speaks volumes. Let’s support and remind each other through being a good example ourselves”