Michael,
I bought an academic copy of MOI with the expectation of using it to build and/or modify 3D geologic interpretation grids. Then using the interpreted 3D geology surfaces I can model the gravity effect of the expected geology interpretation; though I must confess that I haven’t had time to implement any testing yet.
I've attached an image of an interpreted surface of the crystalline basement created using a gridding algorithm that takes faults into account; but there are no programs that allow for simple free form modification of the gridded surface. One of the unique features of geology are discontinuities and faults, therefore geology surfaces are piece-wise continuous with faults and geologic parameters define boundaries between the areas of defined surfaces.
I realize that this is not the conventional use concept of MOI but I am hopeful that you can give some feedback about the concept and/or assist me in the development of one or more workflows to integrate/synthesize and modify geologic data and digitized expected surface interpretations. I believe an implementation of constrained “free form” surface modification would aid geologic interpretation techniques.

Kevin.