I'm doubtful that's possible. A raja isn't going to convert to Christianity after the initial conversion. If my memory is right, after the initial conversion, the Nasrani ceased to convert after the initial conversion, and in fact became part of the caste system and assimilated into Indian society. If you want a raja to convert, it'll have to be by St. Thomas himself. And the ruling dynasty of the time seems to have been the
Cheras, who seem to have believed in some sort of pre-Hindu belief at the time, worshipping a local variant of Kali as supreme. Religion, however, was quite flexible in India at this time, so you could conceivably get the raja of the country to convert, but it wouldn't really hold a lot of say for the locals in this time of religious flexibility.