Part of me is thinking perhaps we need a 'different' warning for a "deep stall" - it shouldn't be hard to detect (stall=true, AOA>X, vertical speed negative and >Y).
OTOH, any competent pilot should know that the correct action to any stall warning is to push the stick/yoke forward (unless ground contact is imminent - certainly not a concern at high altitude). So if they are ignoring the current stall warning, would a deep stall warning be any different?
Perhaps a different method of stall warning is in order - during high stress/workload, the human mind tends to shut out aural 'distractions' - perhaps the "STALL, STALL, STALL" isn't getting through?