There cannot be any Dalits in any other religion than Hinduism. The Dalits who were formerly Hindus, perhaps a hundred years ago or less, were converted to mmostly the Christian faith. The Christian missionaries, amongst other lures, also appealed to their sense of pride, which as Hindus they found crushed under an unjust caste system. If I was born a HINDU Dalit, I would have converted to another religion wherby I would have been no more a Dalit, and thats why many Dalits converted too, so they could be Dalits NO MORE.
So, where have we got Christian Dalits from? Christiany claims to be caste free. Sects, yes, castes, no. So there cannot be any dalits other than Hindu dalits. The question of reservations for dalits other than Hindus does not arise. If any Christians wish to claim the benefits earmarked for dalits, they should properly denounce that converting to Christianity was a fraud perpetrated on them or their ancestors, a promise betrayed - then re-convert to Hinduism.