Hi,
I'd like to add short update.
I've just redeemed 90.000 miles for CUN-DUS-TXL-GDN for 2 pax in J (I don't know why, but companion ticket was not available, so both tickets costed the same amount of miles). I did exactly what you suggested, i.e. indicated the flights in which AA showed availability of J seats and agent booked those flights very smoothly. The lady was extremely nice, so it was a pleasure to spend 30 minutes on the phone :-)
Well, I still don't know why I didn't act quickly enough to book 2 seats in J for our onward journey, when they were available... I don't expect they will appear once again for my dates, so I guess I'll book 2 tickets in economy (perhaps in XL seats) and will wait for possibility to do the upgrade (if such appears).
Bye, Bartek