VanHalenLover: not bad in theory, but I can see multiple account holders taking full advantage of something like this......
They could make it so that the person joining could only be randomly selected, not invited via friends list. That way a multiaccounter couldn't create his own Blitz game, leave, rejoin, etc.
Basically how it would work is this:
If an alternate stroke (or other appropriate) game lost a player, they could vote to wait for the original player, look for a replacement, cancel the game, or forfeit the game. If they voted to look for a replacement, the game would be added to a Looking-For-Replacement Queue (LFRQ.) Once they acquired a replacement player, they would be removed from the LFRQ.
Where does the replacement player come from? If you hit "Join Game" from the in-game main menu, it reads stroke, match, blitz, and alternate, and if you click on one of those, you get a list of options. There could be a little checkbox down at the bottom that said "Willing to be Replacement." If the box is checked when you click "Find Game," the server would search the LFRQ, and if there was a game matching your criteria, you'd be a replacement. If there was no matching game in the LFRQ, it would then perform its normal search.
I'd guess that if 5 credits were awarded anytime a replacement helped finish a round, there would be quite a few people willing to play as a replacement. I think ~40% of games that are currently abandoned or forfeited would be played to the end.