What about having in-game vote kicking and map voting?

Thats what I mean about in-game commander voting.

What would be the interface to vote these things? If you could do it just in the chat to send your vote as a client to the server, the external metagame component running on the server would be able to receive the messages and detect if they were meant as something special, e.g. /vote_kick x, /vote_map y. The metagame component would then handle changing map or kicking someone out the game through a server command. Both commands exist already.

That way it would be again a metagame mode feature which I prefer. Some modes don't have any meaning for voting maps so integrating it hard in the game/server would seem unnecessary. Only reason such features would make sense in-game would be if there would be special needs for the UI to do these things, as, at least currently, defining custom server specific UI externally to be shown for the client in-game is impossible.

I'd say the reasonable choices to achieve a thing like this would be the in-game chat system or being able to click a button/link somewhere which would open a web browser to the server so the server can provide a customized UI for player interaction, if needed.

A chat function would work. Its not the cleanest way to do it, but it would work well enough! Most games use the f1 key for yes, and the f2 key for no..

When you press escape, you have an option to do those things.