You do earn EQMs on BA flights in discount Y, but at a reduced rate. You don't earn any Aadvantage bonus miles, just base miles. http://www.aa.com/aa/i18nForward.do?...ishAirways.jsp

At 6-10 transatlantics/yr, assuming LHR-ORD-LHR, it doesn't sound very hard to keep AA Plat, hence lounge access. Or, if you mean 3-5 round trips, then yes, a bit harder, but 5 x 3950 x 2 = 39,500, so a couple of roundabout routings - LHR-FRA-DFW-ORD etc could probably narrow the gap even further.

Another thought (outside most boxes) is joining Alaska's mileage plan, which lets you book miles on BA TATL (still at measly rates in discount Y) but also AA, AF, KL, DL/NW, LA, QF and CO (at least for the time being.) Pretty good program, too.