I thought you were reffering to cancel ELO for ALL games, not only 2+. Still considering that many games are not playable with 2, and those which are, are rarely balanced for 2. Player base will be much lower.
Yes, and I also say why I can't be sure: because BGA is hiding ELO details for coop games. But behavior I observed give clues that system, or something close, is implemented there. Game is Pandemic, its not secret, you can check if that will give you details you need.You say there's one games that "might" have it, so you're not even sure, but if you wanted to streamline things it would be just as easy to say why not eliminate the holdout. Easier even, given its removing work to deal with down the line rather than adding to it. Hard to say given I have no idea what game you have in mind, which might give important details.
Part of agreeing to cancel was already handled, badly but general principle is ok. Just instead of all or nothing policy, set a border when you CAN cancel if all agree, like below 50% game in progress, or 33% or 25% or whatever value you feel like. That will be enough time to cancel games with people you can't agree on basics, and still far from abusing it for boost your ELO.But cooperative games are very different from competitive ones. For one thing, competitive games have real opponents, ones that actually think, where cooperative games don't. This distinction alone is the cause of one of the boosting issues in cooperative games (everyone agreeing to cancel losing games) that had to be handled. Cooperative games are going to have their own problems and need their own solutions, what works for one group won't necessarily work for the other.
And that's the main problem, the believe that coop games are different. They aren't. Neither in this, nor in most of matters.
System could be same, in basics as one used for competitive games. With 1 major adjustment: not treating coop as something different. Treat coop games as competitive games where score depends only on winning with 1 player (many versus 1): player 0 (game). So games in witch only valid strategy is cooperate with others to do that.
PS: Thinking opponent is not needed for ELO calculation at all. That is not a factor if your opponent thinks or is some sort of automatic process.