Having said that, if the type of deep stall that has occurred here can render the normal stall warning useless due to airspeed < 60kts, would it not be possible to derive a warning from a combination of high rate of descent / Vertical speed together with a high nose up attitude above a pre-determined or abnormal value?
A much simpler and reliable solution is to latch the stall warning once a (fully developed?) stall is detected then do not clear the condition until sensors show the aircraft to be flying (unstalled) again.
While clearly not the "cause" of this accident the stall warning re-appearing when the correct (ND) input was applied could not have helped.