Are we sure he is not asking if they can be intentionally held to deep stall. If that is what he is asking, the answer is yes. Hold the stick back(Boeing) and you will eventually enter deep stall regardless of the backup systems. Is it recoverable at that point? Depends on the pilot. During certification of a 727 hushkit I went through 600+ stalls as the observer. Some were so deep that the indicated air speed was less than 80 kts. During one stall, the right wing broke early and we went about 95% of the way to inverted.
If you want to get real cute, lets start talking about wind up stalls.
