there is a mentality that translates "politeness required: 0" into "politeness required: arbitrarily large negative number" ("no need to be polite" -> "be extremely rude"), and I think there's a nonzero chance of that mentality being present on LW.

Yeah, I've thought of that. If such a feature were to be added, it would probably be good to try to preempt that effect (and other possible problems) with a "The proper use of Crocker's Rules" post, so people will at least be mindful of those pitfalls in advance.