I would suggest to exclude AI from games and especially from the tournaments because AI is stupid and such your teammate is an advantage of the enemy team. I think it should be open place for people to join.

to my knowledge, AI cannot be added to tourneys

AIs in games is a personal preference, if you dont like them, simply make your own game or join games that have no AIs.

he probably is talking about ais replacing booted players

You just gotta work your AI and win despite a disadvantage
http://warlight.net/MultiPlayer?GameID=2479642

Making an abandoned seat in an ongoing game available for anyone seems like a great option to me,
i suspect many will hate it as well but that is why game-creators can decide on things like this when setting up a game
This is where suggestion go:
http://warlight.uservoice.com/forums/77051-warlight-features

Instead of an AI an open place for people to join? Bad Idea. In tournies for adverage players a top player might get that open seat. Or a newbie gets it and gets booted again. There even is a possibility of getting a teammate that is worse than a AI. "Cheating" is easier this way I think, so I would rather not have open seats. Furthermore I think that the AI are not that bad, altough most players can defeat them without much problems an AI can put up a better fight than you would expect.

If you want your teammates not to be booted you have three options (none of them garantees that your teammates won`t get booted, but lowers the chance a lot)

A: Play with a friend: Simple, you know them well, less chance they let you down and once they go inactive you still can make contact to ask them to take turns (or even account details)

B: Join a clan: Clans are not only for competition and making new friends, but also to be sure you have a trustworthy teammate. Clan-members (of your clan, but also from other clans) will not just go inactive most of the time. When you are in the same clan you`ll get warnings before that and you can make agreements about that one tournie

C: Join a top-player (not recommended): Topplayers do not often go inactive and when they do they mostly let the rest of the community know via the forum. This is not recommended though because topplayers mostly have a small group of people who they play with and do not like random teammates as much as newer player do. You may try, but top-players mostly reserve a spot for a friend in a tournie.

My advice is to clean up your invite list (don't do auto add). That way people who get booted will never be invited at all.

You can also set boot rate prereqs