whereupon if I'm playing WOW, I roleplay an elf. <...> If I'm on LessWrong, I roleplay a rationalist.

Or you can roleplay a rationalist elf in WoW. :)

A long time ago, back before I quit WoW, I roleplayed an atheist draenei who refused to believe in the night elf goddess Elune. The catch here is that we players know she actually exists in the setting, because Blizzard told us so, but the characters would have no way of verifying this since she never appeared in the world in person. From my character's point of view, the magical powers that priests of Elune attributed to their goddess were actually (unknown to them) given to them by other, non-personified sources of power followed by other priests in the setting.