If a ship wins a boarding party against a ship with Unique Treasure on board, can the winning ship decide to steal a Unique Treasure from the loosing ship? The Code is open-ended in boarding, simply saying “a Treasure”.
UTs are included with “treasure” in situations like that, so the player losing the item can certainly hand over a UT instead of a standard treasure (unless something specifically prevents the UT from transferring), but you can’t force a UT to be chosen if the player wants to give you some other treasure instead (unless the UT or some other ability does require/allow it).