If you use Part Instance, you can swap in a One-Off version... If I recall correctly you do this via the Configuration.
Logically speaking, you probably don't want the One-Off Version in your "Design" Part Structure because this represents the blueprint, or how it should be done next time (otherwise you would cut a new part number or Revise). The place to record the One-Off is in the As-Planned/As-Built which the Part Instance is meant to represent.
As a sidebar, I have also discovered that you can't do a One-Off version of a CAD Document, which seems kind of goofy to me because you'd probably want to document differences in the CAD model as well as the WTPart. It seems overkill to make a different model number/name and associate it to the one-off part. If the CAD Document ID is different, then why not the WTPart? Which would push the One-Off Version (difference) to the Parent.
