All this discussion of 'forewarning' is to my mind a waste of time. The thing to do is to PREVENT HAL doing something stupid when things do 'off schedule' as he did in the 2001 A340 case. Providing his sticky little electrons can be kept AWAY from any flight controls all will be well. Airliners generally fly in a stable manoeuvre environment and the 'Oh my god' need to pull/push immediately on a control is unnecessary. There will normally be time to pause and assess - no need to 'forewarn' - yet another ECAM/chime'/flashing light to confuse the poor human?
Unless something has fallen off or the a/c is wildly out of trim for some reason, sit on your hands, think about it, leave everything where it was for a few moments and THEN do something - as I say, thisi is fine as long as Hal is not doing something else..
Link - "If one could reasonably anticipate the loss of Pitots leading to the need for hand flying for,say, half an hour, ought one to try to reduce ones flight level by say 4000 ft." - no! No-one is going to do that. In which world did you fly? I suspect most pilots would be thinking hard about "doing a 360 and getting out of there" as a navigator once said..