BOAC, could it really have reached max elevator? Surely in that case the AP would normally have trimmed the stab, to relieve the elevator force. Or is there a flight envelope for the 73 NG with AP on that is different when hand flying?
RMC, on most aircraft the AP will disconnect with a certain force on the yoke/control assembly, releasing the controls in their out of trim condition. Sounds sort of what you had, although your's may not have been as extreme. On modern aircraft though, the AFCS normally sets off a mistrim message before the force gets to the point that the AP will let go. This alerts the pilot that all is not well. Does the 73 NG have such a system?