Just booked an AA codeshare DFW-DOH-AUH and was very happy to find this after a bit of frustration on the QR website not being able to select a seat.

Data Point for my flight in April:
EF showed all the middle quad seats as blocked. AA reservations seemed to see the exact same availability as was showing on EF and was able to book me in a an available window seat that i chose based on what was showing on EF.

Is it worth my time at all to call QR and see if they can assign us connecting middle seats outside of 72h? Any new developments on this front?

Call your airline or travel agent and request they cancel your seat assignment for the QR segments. Then call QR and the agent will book any seat.
I booked 2 seats on a flight DOH-YUL through American Express Travel.. Amex Travel assigned 2 single window seats as they do not have access to the center seats.. QR could not change seats assignment until I had Amex Travel cancel my seat selection. Voila, QR was able to assign seats. No problem.