[RyanCahoon]

Inventor does have a Demote to subassembly operation to create a new subassembly from a collection of components, which seems kind of like LabVIEW's "Create subVI." The difference between this and what you ask is, of course, that this only works on the assembly level and not in parts. Would it work to combine this with in-assembly part creation? You would have to create a new part whenever you wanted to add a feature to your "part" and your design would consist of a bunch of part files in the end that may or may not actually have meaning in your final design, but it seems like it would achieve what you're looking for.

--Ryan