It would be easy enough to install a strain guage in the critical wing areas on this airplane....the wing root just inboard of the inboard engines and at the wing join just outboard of the outboard engines.
You could feed the data in to a logging device for later download, however; the loading is still a function of 'g' loading on the airplane, so..if my old memory serves me...the 'g' meter is located on the upper corner of the F/O's instrument panel.
The fact of the matter remains in it all...if the drop run is fast, down the side of a hill and the aircraft is acellerating and the load is dropped...there is a huge change in stress.
Another consideration is that in a fire area, the turbulance is awful. The heat generated by the fires, combined with any wind gives you quite a ride. So, there are natural problems aside from induced stresses for this airplane as well.
Remember that LAPES (Low Level Parachute Extraction System) is a low level, steady state scenario. The aircraft is kept at a low constant speed....the altitiude is constant and the load is then extracted. All you need to do is watch the pitch change and altitiude change as the load goes out the door, to see what kind of a difference weight change makes in this aircraft.