You might find this approach helpful: Dynamic 3D Object Manipulation for surgery simulation - #2 by lassoan. The video shows how you can create a smooth nonlinear warping transformation via paired sets of markups points. You could experiment with
- generating points at on the ventricle surface and at the distance at which you want no motion to occur
- duplicating those points, and then displacing the ventricle surface points by the amount you want the ventricles to dilate and in the direction normal to the ventricle surface, while leaving the other points umoved
The resulting transformation should (to a first approximation) show no motion at your fixed points or outside them (provided you have enough fixed points in enough directions), your specified displacement at the ventricle surfaces, and some sort of smoothed interpolation of partial displacement further out. This may be good enough for your purposes, or you can try to refine it by adding more points along with the displacements you want to force at those locations.