Interestingly, it allowed me to check in about an hour ago. The boarding pass covers both flights, including the "connecting" flight. At the time it checked me in, the "connecting" flight was 40 hours in the future, and, not surprisingly, I was shown as number 1 on the boarding pass to check in for that flight (and 143 for my TATL flight which was 6.5 hours before departure at that point.)

I am wondering if the problem is that the system cannot do a check-in for only the first of two or more connecting flights because, unlike some other airlines, there is one boarding pass for multiple flights. And while the system will let you check in for a connecting flight more than 30 hours in advance, there is still some outside limit as to how far in advance one can check in.