Pomegranate juice ceviche
Could you use a combination of pomegranate juice and lemon juice? 

A recipe on the POM web site uses grapefruit juice with pomegranate juice.

A quick search reveals that most (all) ceviches that use pomegranate juice also use another acid (lemon, lime, grapefruit).

If it were me, I'd try using only pomegranate juice only on a small batch of fish a check how it goes.