Winkler foundation boundary condition?

[ricitron]

Is it possible to implement a Winkler foundation boundary condition for the bottom of a 2D box model? It would be a variable force on the bottom boundary equal to the boundary displacement times the gravity and mantle density. The idea is to simulate the flexure of the mechanical elastic lithosphere when a load is placed on the surface. Would this be possible with a the current boundary options or does it require a code modification?