-snip-
I remember I was testing out a planet mod I was making for a DM, and I had used this method and I didn't have any of the problems you stated, for example, I went into a zone with regular gravity, hoping that it would be applied, but I still was in zero gravity from the zone I gave the player.
Also I am certain the zone was smaller then the player object.
Updating the position frequent shouldn't be a problem, since its just changing the zones location.
I don't see why come there isn't a global gravity variable, it would be useful for some gamemodes/builds.