I would say that Javan rhinos could be successful, and we have to remember that PL would have had better success with Sumatran rhino had they had better specimens - from what I hear they were the first to crack the husbandry of the species.
Yes they were only 'unsuccessful' with the Sumatrans as far as breeding was concerned and that was quite probably simply because of the condition/age of both females they were allocated. They maintained them in good health(the male at least) over a number of years. But I think their perceived 'failure' with them- even if not their fault- may be sufficient to preclude them ever having Javans- but who knows?