I believe the navy does use negative pitch, to hold them on the deck. The EH 101 has up to -10 deg negative pitch (if memory is correct).
Wouldn't using negative pitch overspeed the rotor? I thought the collective was used to hold Nr around the 105% -110% Nr in auto (dependant on aircraft type), as this sort of speed gives enough stored energy for the flare/landing. Even during an auto, I didn't think the collective was fully down, except maybe initially. If you were caught by suprise (no warning of engine trouble, and you had to put your magazine down before grabbing the controls) and the Nr decayed a bit too much, I suppose the negative pitch scenario may help to give you bit more Nr, but then once established I don't think you would stay in that position, but you would return the collective to a positive pitch position to control Nr.
As ever, I await corrections. I may have experienced quite a few autos (all eng on), but not being a pilot have never controlled one.