Sorry to bring back an old thread, but I just found something that seems to apply to this topic:
Autodesk Inventor Multi Body Assembly Layout.
Not sure if this is the same as the sketch blocks that were referenced previously, but it allows you to make all the parts of your model within one Part file, then break them into individual parts within an assembly at some later time. Still not 100% because you have to split the features into new bodies when you create them, but it seems to be closer to the desired work flow.
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