Couldn't you take the BGA and the heatsink loose and perform this test? If you could think of a manner in which to hold the BGA when trying to pull the heat sink off then you wouldn't have to play with a soldered BGA.  Word of warning about the load cell of the torque tester, I had done some testing where we were testing the sheer strength of SMT adhesives and used a torque tester.  We blew the load cell of the tester by over stressing the tester beyond the max limits.  It turned out to be an expensive sheer test.  

Kathy