I did find an answer, at least somewhat. After spending hours searching through the customs bureau's website, i just gave my local office a call (duh!). Who would have thought, they were helpful.
In fact, they came up with a great answer for the particular case I was considering - a mirage with PD canopies. Since all of these components were made in the U.S., this would constitute a 'return of American goods' and therefore would not be dutieable at all! This applies only, however, if the good has not been altered or improved while abroad (i.e. repaired or something).
Still no idea how much duty a system would incur that was not made in the US. The officer also didn't know.
Sadly enough, after having found all this beautiful information, the ad for my dream rig was already pulled from the dropzone classifieds. That's irony, I s'ppose. ---
The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt.
~ Bertrand Russell