Frankly, I directly searched for some forum area discussing this, and I only have skimmed posts aside from the first. I am going to take a hardline extreme stance on this.
There should be no ridiculous punitive measures against leaving the character selection. The game hasn't started yet, and if there was a convenient, no hassle way to do so, the game could only be improved by quick avoidance of trolls, belligerent types, auto-lockers, and poor team composition. Having the foreknowledge that any of their teammates could leave the lobby would be extremely good incentive for people to be considerate for a change.
There is no justifiable reason why one should just adapt and make a null subject of an intense pain. Riot can be bothered for just a moment to improve interface and community interaction. They don't need to always be working on skins and characters.