The title "BG3" was just an excuse for using a famous licence. The game itself had nothing to do with the Bhaalspawn saga, and even less with the city of BG. It's better to call Jefferson by it's TRUE title, that is "The Black Hound". It could perfectly fit in an expansion or module for NWN2, even if it isn't connected with the OC. Look at SotU, it didn't follow the story of NWN1 and your character started at level 1 and was supposed to reach level 15 by the end. Obsidian could use the same approach for Jefferson, and maybe even create another expansion, in which you will be able to import your character (just like HotU did).
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I know, they used the BG name on those horrid console games that had nothing to do with the series.