It's an okay idea, but Source Collars glow brightly with magic, and broken ones do not, so it would seem to be incredibly obvious when a collar is not working. I could understand why Larian might not implement this bluff check.
I would be more bothered if Larian didn't implement a "[Mislead] The one who stole your Drudanae is dead now" bluff with Griff. They probably won't do that either.