OK,
sorry, I don't want to tread on toes, nor let the story get too far from the facts.
The reference to the Manual was ref the Hyd system failures and the PES.
My manual says nothing about using the autopilot as an attitude hold, I was just suggesting that with no stab trims whatever (a 757 is in this situation with Ctr and Rt Hyd systems inop) the autopilot will engage for a while. Occasionally the thing trips out. My (and others) analysis of this is that when a stab trim input is required (attitude change) the thing trips out - thats what the sims do anyway. Just using the autoplit as an attitude hold for a short while can be a great relief when a TRE is breathing down your neck and you can get a respite from hand flying while briefing etc...(but be ready for it to drop out!).
I agree the autopilots do engage on the ground (I tried it today). I would be interested to see what happens if you try your test in a 757 with the Ctr and Rt Hyd systems off and use the autopilot and use the elevator. I suspect the stab will not move to cencel out the elevator input.
Now to the point I was trying to make earlier. The difference between the 757 and the 767 is the PES; which allows stab trim to be used at 1/4 rate when the left and ctr hyd systems are inop. My manual says is only possible to use the electric trim at about 1/4 rate, automatic and alternate trim are not available.
The 757 has a PTU for entirely different reasons.
Hope this is clearer.