Would it not be a smarter system if you have a player disconnect, you have the option to surrender before the 20 minute mark? Quite frankly I don't think anyone wants to waste 20 minutes of their time because people have to quit for unknown reasons at the start of the game.
Possibly apply an incremental system based on the number of disconnected players, that will reduce the time of surrender by a set amount of minutes. I say this because even if a player has returned before the 20 minute mark; I would say your chances of victory have decreased quite dramatically and the returning player is always a liability.