"Testing it on the ground may allow it to work as the elevator can probably put in any demand, only when the stab trim tries to operate will the autopilot decide it is not going to do it and trip the autopilot out. "
I'm not 100% sure I understand what you're saying here, av8er, but....
(On the ground) I did try operating the L A/P with L hydraulics and the stab moved to cancel the elevator input, so I assume(!) the Right A/P (with only right hydraulics) would also detect elevator movement and (try to) send a command to the SAM to move the stab.
"It is actually possible to use the right autopilot as a sort of attitude hold with the left and ctr hyd systems failed, but it will trip out when stab trim is needed (ie any major attitude change)."
If it states so in your (767?) manuals, then I will accept this (My manuals are not entirely unambiguous).
Thanks.
Rgds.
Q.