Short-check to T5 or bag unload
I'm flying AA SEA-JFK then BA JFK-LHR-IST. I have a separate ticket on TK at IST. I have one bag that I want to leave in LHR left-luggage so that I don't have to get a visa and go landside at IST. It seems I have two potential options, short-check the bag to LHR, or if AA won't let me, request my bag to be unloaded at T5. Assuming AA won't let me short-check, can anyone tell me roughly how long it's likely to take to get my bag unloaded by BA at T5 and what the procedure is?