An idea, that may or may not help prevent leavers in game.

I think, that it may be possible for you guys to implement a little warning when you go into game, but it would only show up on one circumstance.

Maybe you could have a warning that says something like "Warning! A summoner in your lobby has recently disconnected from an active game."

It would give a heads-up that you may have someone disconnect in the middle of your game. I think this would be useful, because most 4v5s are kinda rough, and in ranked it might be worth it to just lose 3lp than to lose ~15 AND waste some time. Maybe if someone dodges while the warning is active, they can lose like 2lp instead? Just a thought.