It is true that you need to be aware of the "earn by" aspect. I should have noted in the original post that one should be careful to input the back credit requests in chronological order (at least up through whichever flight triggers the award). As long as you do that the next earn by date will end up exactly where it would have been if you'd been supplying your RR number with your reservation as usual.
One other caveat is that my experiment did not involve going back months. I see no reason why it would work any differently for older back credit requests, but it would be nice to have explicit confirmation from another test.
rove, yes, check the language of the offer but frequently promo credits will not be earned with this technique; consider whether the promo is worth $50 out of pocket before deciding how to proceed.
bryanw, as explained in the FAQ the system retroactively restores credits that rolled off when earning activity would trigger an award.
rrgg, thanks for the info on having an RR number removed from a record; it's good to know. When I did this experiment it was for "pure research." At the time I thought it was just the first of the two awards my wife needed for our anticipated trip to Hawai'i this month. I might have tried to have her RR number removed from her second trip if I had suspected we would end up not making the Hawai'i trip.
Thanks for the kind words, Firewind. I'm indebted to this community for many great tips, especially the Continental-AGR-Choice points transfer (we're staying free several nights in Europe; see my post on the Choice board) and the 8 credit SFO signup bonus, which two of my wife's sisters were able to use.
ftn