From June Hanley, head of the music committee:
You are free to choose parts from a four-parted tune; your choice of which two needs to make musical sense, naturally, so this is usually part 1 and either part 2 or part 4. Also, there are lots of good two-parted tunes that aren't heard very much, so those are good to look at as well; you can always transfer them into a medley in the future.
So it looks like there's extreme flexibility for the MSR. Just wanted to pass along that update. We're now taking suggestions for tunes from anyone. Keep your ears open.