Air China flies a 5th freedom route YUL-HAV and back (feeds from PEK). I checked some random dates and found fares for ~#350 in Y and ~$850 in lie-flat J (773).
Theoretically this should be bookable for 35/60k SQ (UA can't book to HAV).
Cuban visa is included in the price and gets picked up at the gate.
Added to wiki.