The answer to this question says that movement speed doesn't let you piledrive your target into the ground - no bonus damage to hitting an enemy into the ground.
I'm interested in finding RAW for a similar case: a player grapples an enemy, picks it up (as if to move it) and slams it into a nearby wall. It seems like the improvised weapon rules might be applicable (a dead enemy is a valid weapon) for attacking the wall, but that deals damage to the wall not the enemy. Furthermore, the natural consequence is that the player hits another enemy, rather than a wall/ground.
If there are no RAW, and there don't seem to be, I'd like some advice on a house rule. Balancing verisimilitude (hitting two people together certainly hurts them both) with balance (essentially picking two targets for each melee attack instead of 1) is the key feature.