You get to go AI about 20 more times before you're accused of Cohenesque play.
Seriously though - I think that's a big problem with 12 hour games. Early on it's great, sure! Get those 2-5 minute turns with no action out of the way faster!
But the game runs on, and ... It starts to take half an hour a turn. Then an hour. And if you're on 24 hour cycles, that can be hard enough some days - but twice a day???
Personally, I'm more and more inclined towards 48 hour games nowadays, because I have seen this happening. Even better, I think, is if people agree beforehand to shifting timelines. Maybe 12 hour turns for the first 10 turns, 24 hours after that until turn 35 or 40, and finally settle in for 48 hour turns. After all - it's much easier to find an hour or two once in two days; and it lets you take a quick look at the turn as soon as it cycles, and then ponder the situation offline, while commuting, sitting in a bar, sitting through long meetings, etc.
None of which has to do with how to minimize the impact on a game when one of the major nations goes AI, but ... Had you considered asking if the other players would agree to a slower pace? Quite possibly some might be in the same situation as the game started to demand enough time to almost qualify as a part time job.
One huge thing that could be done by Illwinter though : Add some rudimentary diplomacy, or at least "allied / friendly" settings.
If, when you go AI, you could tell the AI which nations you were allied with, which you were at war with, etc, then at least it wouldn't have quite the
huge impact of someone's trusted ally going AI and all of a sudden their previously safe border is getting devastated by the 'ally'.