I have not been able to find any pomegranate juice that did not have preservatives in it (Even over priced organics), but this sounds tasty.
You may need to be ready to back sweeten with some artificial sweeteners.
In my limited experience pineapple and pomegranate can lead to some harsh acid taste once all the sugar is gone.
Otherwise, it sounds like a pretty nice drink, please let us know how it comes out.
In Primary: Belgium Chimay clones.
In Secondary: Braggot, pale ale, end of the world white.
Conditioning: Mead, Cider, braggot, Belgium Wheat.
On Tap: Clones, Chimay Blue, Red, Porter, malted cider.
Bottles: Far, far, too many to list.