Temporary AI
Instead of having to boot someone or wait forever, how about having a limit after which the person is turned into an AI for just one turn. That would let the game continue and olny penalize a slow player moderately but not remove them from the game. A setting may be 2 days and then vote to either boot or temporary AI the player. Also, instead of 1 turn maybe set it so the player could just opt into the game again when they return with the AI just replacing until they do the opt in. That would then be the same as a booted player becoming an AI, but allow the player to return.
Now that players can come back from being an AI, this essentially gives you what you’re asking for — someone can be an AI for a few turns and then return to being a human. See the blog for the announcement: http://blog.warlight.net/index.php/2013/03/site-update-1-16/