Bumping this as I have this request as well. Enabling tournament mode allows you to choose teams, but also requires each team to "ready up" for the game to begin. There are several arena pubs that allow you to choose teams while still running in a pub mode and they both have mp_tournament 0 in their cvars.
They must be using some sort of plugin, but it doesn't appear to be publicly available. I'd love to see one though as well.