I was also relatively sure that neither Minsc nor Jaheira are romanceable, but purely because they are D&D/Forgotten Realms canon characters now, so the characters (IP-wise) belong to WotC.
In the Pathfinder games by Owlcat, Amiri in Kingmaker and Seelah in Wrath were not romanceable because of that reason. but maybe WotC handles things differently than Paizo.
