So are you're saying that a book I may have purchased from another site ("Girl with the Dragon Tattoo" for example purchased from the Sony site) can be synced if I convert it to mobi for my Kindle, as long as I change the metadata values? That would be really groovy, if I understand correctly!
That is correct. But only the furthest read point.
Unfortunately, calibre doesn't allow you to add (or manipulate) the 113 or the 501 EXTH metadata records. Mobi2mobi is the only tool I've found that will allow you to add
the proper records to an ebook that doesn't have them.