Wirelessly posted (BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)

"However, when I tried to help a friend to get a J award from EZE to DTW, the BA agent insisted that the domestic segment had to be in Y."

In the future, I would recommend pointing out to the BAEC agent that the international leg is longer than the domestic leg. If that doesn't work, hang up and call again. The rule I mentioned was posted in the BA forum a few months ago by a BA reservations agent. And there was a recent post indicating that someone was able to get PHL-DFW-MIA-BGI-MIA-DFW-PHL itinerary (including stopovers) with the domestic segments in F, on a 70,000-mile BA J award.