Well the naginata was not designed to be used on horseback rather a deterrent to mounted combatants. Now on to the subject at hand I firmly am of the mind if there is no rule it is DM discretion, that being said yes the game was made to emulate real world physics and fantasy why would you want to penalize a player for thinking outside the box especially when there are no rules that say if wielding a light weapon you can't atk a small creature or low to ground med creature in mounted combat. Also they would need to introduce facing into the game because you could not atk a creature directly in front or behind your mount and that is a level of complexity that is not needed in the game. I say let it go this is not something that breaks the balance of the game and the game does not need extra rules to complicate a non balance issue.