Terrain, alteration of shader map at runtime.

If we build a shader using vertex deformation and reference the deformation value from a texturemap and the UV corrdinates from (say) the player’s position, could we apply that shader to the terrain and paint into it at runtime, thus allowing the player to leave a trench behind him as he goes, bulldozer style ?

Or is there a better way ?
We’re quite happy to crawl into the guts of the renderer and shred the actual code itself if that’s the better path…

PS. no way to deform a heightmap based terrain at runtime, Why the badgers not ?

Many thanks,