Adding a Nested Family to an existing Family implies accessing particular workplanes, references (edge, surface, point) on existing geometry and the added Family, and associating new or existing instance/type parameters. Is this how you see it as well?
Rhino.Inside.Revit is great for controlling these parameters, but would require particular scripts to be developed in each Family>Nested family creation. Not exactly a fluent workflow to generate programmatically in a general context.