Hello Mr Palmer;
My own A330 simulator trials of recovery after the stab trim was full nose up and the stall fully developed was that a prolonged nose down input did help—but only initially. However, as the angle of attack reduced and the speed increased, apparently the full nose up stab was more than the elevator could overcome and the airplane pitched up regardless of the nose-down stick position.
With the stick held in the full-ND position, did the THS never begin to move towards normal, (from -13 to around +2 or so)? It should have, so that's a bit odd that it didn't move.
In my own sim trials the elevators were sufficient to initially achieve about a 10-deg ND pitch attitude, and the THS always began returning to normal when the stick was held full-ND. I agree that recovery was probably not possible unless/until the THS returned to a normal setting.
A manual setting of the THS was always available to the AF447 crew but it would have returned to the NU position once the manual rotation of the wheel stopped - by that time, they may have got the wing unstalled. In our trials it took about 20,000ft, about 45 seconds.