I don’t know how because MRJ was filming in London the entire time OUAT was filming their finale (and had the longest, scraggliest beard known to man!) but I guess they could get one of the Young!Bae versions?
Secret last minute scene filmed weeks after filming had wrapped? Or perhaps just a voice over? Not sure how either would work, but if there was no appearance from Neal/Baelfire at all, you would have thought they’d just say “No” or a vague “can’t say” that doesn’t mean anything. This answer is more specific.
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