Yeah, you're asking for a sort of Boolean Displace function.
Its easy enough to calculate the volume to displace but the math involved in where to displace it gets a bit ott. Problem is you'd need vector to distribute the mass and force to control how far/thick to distribute it. ie All on one side or equally spread around and then whether thick/thin close or far spread.
Its a neat tool you're describing. It'd allow you to run a cartoon figure into a door and have it auto splat for you or a heap of other uses.
However, apart from manually as you said, I cant think how to do this in AC3D or in a bunch of other much more expensive programs.
