you could put a trigger volume over that hole and name the vehicles different names, and then if the vehicles go into the trigger volume which is on the hole a script will make a transparent object which is in a fixed position so the vehicle will just ride over the object

or

the same method with the trigger volume but instead of the script making a object cover the hole you could make another BSP of the same map but have a invisible texture over a plane which is over the hole, so a script can then switch BSP's instead of putting a object over the hole. And the when the vehicle goes out of the trigger volume is switches back the BSP, this will allow other players to get down the hole at the same time because BSP switch doesn't sync unless made to. So only the player in the vehicle will switch BSP's.