Owain seems to have a decent description of some of the factor involving the THS and its effects upon all the moment arms.
I refer all to my graph of the Viper pitch moments ( will find the post later, as on slow landline connect now). Granted, we flew at a very aft c.g. compared to the 'bus. But the fact that the jet could settle into a fairly benign condition at 50 deg AoA with no violent pitch changes and a very good directional condition ( no spin), and only small roll change was a surprise to all of us.
Our HAL cut out all pilot pitch inputs via the stick once AoA was above 30 deg or so. HAL also took over rudder to help prevent a spin. In short, once in the deep stall we were observers. But we still had NU capability if we could only gain direct control of the HS. So they gave us the "manual pitch override" doofer. It only worked if AoA was above 30 deg, but it allowed us to "rock" the jet outta the deep stall.
Show us the pitch moment graph!!!! And I realize that BA did not get a 'bus into a deep stall to get the data. But it would seem to me that they could calculate all the moments and control surface effects.
I have a feeling that the THS could have helped the pilots if they had manually moved the doofer to the nose down angle. I don't have the control logic for the THS, but looks to me from reading all the manuals that it simply moves the HS to minimize deflections of the elevator. A neat idea, and doesn't need AoA or airspeed or ..... So a constant pitch input could result in moving the HS to the limits, especially if speed was slow.
On a personal note, and being a pilot, I feel sorry that the crew was presented a condition that just didn't "compute". I think that if any had seen films of the Viper deep stall that they would have tried something after about a minute, anything, do something different fer chrissakes. Would love to find film of our deep stall, and you would be surprised how easily the jet settled into the stall.