It would be nice if we had something like the player-killer bit (on multiplayer games, something you can set that means your character can kill other players and can be killed by other players), that meant "I can handle criticism", and that would show up next to your name on every comment you posted.

I'd support that. I think I once suggested something similar, adding a Crocker's Rules bit. What I'm wondering, though, is what levels of politeness would then become expected toward people who choose not to set the "I can handle criticism"/Crocker's Rules bit. Right now, any such norms are informal and implicit, to the extent that they exist at all, but if we added an option allowing people to explicitly accept bluntness, it seems like we'd need to clarify what the norm should be for people who don't enable that option: whether not setting that bit should be taken as "please do not be blunt to this person", or "please be polite to this person unless it's seriously impeding communication", or "just use normal social discretion, if you have any", etc.