There are a lot of people that like to fill. Team Builder doesn't allow for people to fill. Team Builder also has a problem in that many Supports (and to some extend, Junglers) pick their champion based on the rest of the team, and Team Builder doesn't let them do that.
I think for it to work, you'd have to have people opt into the system, and can only be opted into if you're in good standing (not getting chat bans/trolling/intentionally feeding). Whenever there's an AFK that they could fill, they get a Game Invite style notification that they can accept or reject. Accepting instantly sets them up to replace the other person (and if Riot is feeling generous, lets them change Summoner Spells, Runes, and Masteries), so there's no Queue to play. This could be a big incentive to join the program, playing faster than you otherwise would.
Once in game, I think it'd be fair to have the option to do a full-price sell back items. I don't think this should be mandatory, because there are items that stack, such as Tear or RoA, which would loose value for being re-purchased.
It's what happens after the game that's the hard part. If the AFK takes full responsibility for the loss, there's a possibility the replacement could just troll because it doesn't effect them. If the replacement gets full LP for a win, then why would they want to play a game from the beginning, since they have less risk and the same reward for being a replacement?
The best I can think of is that you'd want to have a system that forces you to play your own ranked games in between being a replacement. I don't know if it should be every other game has to be one of yours, or if longer periods work fine as well.
I think the replacement should lose 0.25x the LP they would have if they lose (they're taking over what could be highly unfavorable situations, after all), but gain between 0.75x and 1.25x the LP if they win. The win amount could be determined by how vital it was that they took over for the champion, how long the game had been in progress, etc. If the replacement AFK's or leaves, then it's a strike against them being able to be a replacement for future games.
In the event that the AFK comes back, then they need to be able to resume the game. They already picked their Summoner Spells, Runes and Masteries the first time around, so those are still locked in. The replacement gets a notification similar to the Surrender box, letting them know they have 30 seconds before they'll be forced to swap back, and they can click the button to swap back sooner. This lets the replacement finish up any last second fights and try and recall. The recall happens automatically when time expires.
The AFK player takes over with the option to fully sell back all items that the replacement purchased, but there is no change to the sell back price for their old items. The store would ideally be modified to handle splitting up an item, so that the AFKer could get their stacked Tear back and upgrade it into what they actually wanted.
In these situations, the Replacement gets some LP and IP for the time they spent. If they barely got there, then they don't get much, if any. If they were there for most of the game, they get most of the LP/IP
NOTE: there are probably flaws in the idea as I've outlined it, but I think it's a decent starting point to see how such a system could work.