I'm not sure if this is applicable to what we could consider "canon," but there was an X-Box game released several years ago based on the events in
The Thing that actually shed some light on what happened.
The player takes control of a search-and-rescue team sent to the area to look for survivors and investigate. The first scenario of the game takes the player to what is left of the base camp from the film (complete with the icebox alien thing, Blair's hidden spacecraft, MacReady's recorded message, etc). Anyway, included in this area is the awning from the end of the film where MacReady and Childs share a bottle of whisky. Only when the player arrives, Childs' frozen body is sitting upright in the same position the film left him, and MacReady is missing.
MacReady does make an appearance, however. During the final boss fight at the end of the game, a helicopter appears and you jump on in order to shoot the big-alien-monster-whatever-thing from above. Your character asks the pilot for his name, and he responds with something like, "R.J. MacReady, helicopter pilot, US Outpost #31." Pretty cool.
I think I even remember reading that John Carpenter lent his voice and likeness to a character in the game.
Also, according to IMDB's trivia page for
The Thing...
At a horror convention Q&A session in 2008, Keith David (Childs) was asked if he ever knew who, at the very end of the movie, was infected with the alien. He smiled and said, "Well, I don't know about (Kurt Russell), but it sure as hell wasn't me."