I don't know. Who's going to call him mid-season? Any GOOD team is going to be scared ****less of adding him to the mix right before going into the playoffs. If the Pats or the JETS are 9-3, are they really going to risk bringing him into the locker room? And, any bad team, isn't going to want to bother (and he's not going to want to go there, anyway).
So, maybe New England or the JETS, if there's a major injury to one of the starting wideouts. Otherwise, I don't see any other team taking the gamble, not after the Minny and Tennessee disasters.
It's also a lot harder for any football player to get added to the mix later in the season, without knowing the plays or having any chemistry with the QB. It ain't the same as adding a guy late in the basketball season. Hell, even Vincent Jackson, a helluva good receiver, had a hard time getting integrated back into San Diego after being out ~10 games, and that wasn't even trying to integrate into a new system or anything.
Never say never, but I'd place the odds on Favre attempting a return as greater than those of Randy Moss. I know that doesn't really cut down the odds much, but I'd say the odds of Moss playing again are ~25%.