@Emre
May be something like a single entity of a bunch of rocks scattered on the Z=0 plane that has their center XY offsets saved in the secondary UV coord set. Each copy of the entity has its own rotation matrix so each rock can be rotated around each own center but all together on each entity. A scalation of the down vertices in reference to its height expands the mesh and can look half-buried. If you use the same texture, orientation and size of the ground texture into the down part of the rock entity, it will look fully merged.



*edited

A XY offset of the down part in reference to its height could emulate an orientated accumulation like in slopes or the effect of wind.



When the gradient is 0:rock <> 1:ground, multiplying the ground color by the gradient could be a good looking effect.