Err,
I may have to eat humble pie here. I tried to see if the a/p would trip on both 75 and 76 the other day, (it's nice to be able to play with the a/c from time to time) whilst on the ground with the stab hydraulic systems inop and after 15 mins of operation during which the elevator went to max pitch up (I selected v/s climb), the a/p was happily engaged with EICAS caution for autopilot and stab trim.
So it seems it's back to square one, as I can't see why it should trip in the air and not on the ground.