N-player rock-paper-scissors variants. They generally involve everybody standing in a circle facing inward shaking their fists three times and chanting in unison, and looking back I feel like they do have a community-building effect. But they bypass the filter because they're competitive, and are presumably appealing to LW people because they involve memorizing a large ruleset and then trying to game it.

In a similar group of my acquaintance, DDR (Dance Dance Revolution, not the former East Germany) serves this role fairly well, albeit only pairwise.

I imagine yoga can work this way for people who don't also have a horror of the reference class they file yoga in.

Large-pattern juggling. Takes a lot of practice from everybody, though - well more than most social dance.

What sort of synchronized group movement or synchronized group voice or both would bypass the Cthulhulian-horror-of-conformity filters?

Something sufficiently geeky.

for example

I'd have a hard time presuming anyone to be completely rational. But I'd have an even harder time understanding why I shouldn't point that out to someone who (presumably; due to them being here and all) wants to be more rational.

About your second point: I'm probably a bad choice for identifying your conformity filters due to the rather big amount of time I've spent at salsa and tango courses. Time which takes gargantuan proportions when contrasted to the awfully little time I've spent in Cthulhulian sects.

The minister's cat might play this role, although people do get kind of frustrated with it.