Spring RTS Engine

Say I am playing vs a Lua ai like Craig, can a spectator take over that ai player?

Correct me if I'm wrong, but isn't Ai, just consider to be another player in the game?

An AI is not just another player, but a Lua AI team should be possible to overtake. You can also "partially overtake" the AI with cheats as-is, e.g. via /team N or /godmode.

Spring.AssignPlayerToTeam should do it.

ah, so will the ai try control things once a player has taken over?

Yes, you'll have to remove the AI somehow as well.

Is there a way for this or does it have to be part of the AI to step down?

I'm not sure, AIs are something I never got into.

The luaAI would have to be told to stop controlling the units. I doubt any luaAIs come with this feature but it sounds easy to add in. The simplest way (which allows the AI to be restarted) would be to replace Spring.GiveOrderToUnit with a function which checks whether the AI is active and blocks the order if it is not.

Sounds a bit like a hack. Wouldn't it just be possible to add a shutdown button into the AI?

LuaAI can use the PlayerChanged call-in to check if someone joined the AI team to disable itself.
The gadget that calls AssignPlayerToTeam can also notify the LuaAIs itself through GG.