Problem solved (there never was a problem, actually). I have a slightly older quicksave that works, so SOS has initialized properly for me at Umar Hills.
For the save game that has the issue, I figure I went to Umar Hills with the two letters, but because I was busy doing other things on that map (i.e. clicking around), one of my mouse clicks may have interfered with the SOS script.
When I was testing older save games, once I had to wait 20 seconds for the scripted SOS message to appear once we arrived in Umar Hills, but it worked.