I would be reluctant to do this even if the boat was on the hard but maybe I'm conservative. Think of the mast as a giant spring under some moderate compressive loading from the stays. If you've been onboard during moderate winds (and no sails raised), especially 15-20+ knots, you've undoubtedly felt the mast "pump"(vibrate)...and that is occuring when it is properly supported. Remove the midpoint support, and should wind blow, I think you might experience severe pumping that could be damaging. If the boat were in some giant building with no air flow and no wave induced motion- then no problem, but I doubt that's the case for your winter storage.