Here is my further data point. I was trying to book PHL-LHR. The original availability was PHL-CLT-LHR, with a six hour layover in CLT. When I looked on AA.com for Business Milesaver availability, I saw better flights (for me from PHL-DFW-LHR. It was better for me because I wouldn't lose a day of work and the TATL flight would allow me enough time to actually sleep. At the recommendation of a BE agent, I placed a hold on a booking.
However, Meeting Services could not convert it to a BE booking although both are "U" class. Then I called the EXP line to see if there would be a different response. The phone call took 40 minutes, and the result was that I was told definitively that although both book into "U" class, that they are, in, fact, different "pots". AA Milesaver availability is not the same as Business Extra availability.