To explore and perhaps clarify some of the effects that would be acting in the proposed configuration--
We consider that the engine, transmission, axle, and other related attachments act as a unit, have a particular mass (weight), and for dynamic behavior that mass can be considered to act at the “center of mass”. Since, indeed, the mass is not all concentrated AT the center, the assemblage also possesses a moment of inertia in the longitudinal and vertical (pitching) plane, which is only important if the assemblage is subjected to an angular motion, that is, in the pitching direction. A small moment of inertia would indicate that the object could be “pitched” easily, while a large moment would require more force, or more correctly, more torque to rotate the object. Turning a set of barbells about its axis is easier than rotating it about an axis perpendicular to the bar.
So, if we consider the pivot point to be in front of the engine, and consequently in front of the center of mass, when the axle strokes upward, it is simultaneously lifting a proportion of the mass by leverage about the pivot and rotating the assembly, also about the pivot, both actions of which require force to accomplish. That force is then proportional to the “unsprung weight”. (No mass, no moment, then no unsprung weight).
Scenario 1: The pivot is located at the center of mass. In this case, the axle movement is only required to rotate the assembly, not lift the mass, and the force required is proportional to the moment of inertia.
Scenario 2: The pivot is located far in front of the assembly. In this case the axle has to lift the mass virtually the same distance as the axle travel, but since the angular motion is essentially zero, the moment is of no consequence.
Thus, the ideal pivot location, which would minimize the effect of unsprung weight, is seen to be a consequence of the particular mass of the object, the location of the center of mass, and its mass distribution, or moment of inertia.
In most cases this would probably be somewhat in front of the mass center. However, worrying about its exact location is likely more of an academic endeavor than a practical one. Perhaps more important on the salt with its lower coefficient of friction than on a pavement course. Appropriate springing and damping could probably make any set up workable.