Not functionally possible.
UnitFrames must be assigned to a strong UnitID, and all UnitIDs are relative to the player. There is no way to create UnitIDs for hostile units other than the delivered options of the boss, arena, targets, or focus UnitIDs. Until Blizzard provides UnitIDs for the other team like they did with Arenas, such a thing is impossible. You can not create UnitIDs, only Blizzard can.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.