I would certainly be interested in seeing this, but there would have to be some considerations and limitations. Either the team with the d/c or afk has not yet lost an inhib or at least a nexus turret for example, because I would be pissed to join a game in progress where the enemy team is already on the nexus and I've lost LP before I even had a chance to impact the game. There should be some cut off point in time as well (say, 35 mins just off the top of my head) due simply to the fact that the new guy still wouldn't have time to earn the gold to be competitive once everyone else is full build.
There should also be an opt-in or opt-out option as well, and a warning that you will be entering a game in progress before you accept. I could also see how this would be rejected as doing so would automatically end any chance of the afk actually coming back, so there would have to be a window at least to give them the opportunity.