The main problem as I would see it, the database was not designed to link religions across islands. Without actually seeing the database this is all speculation, but it is likely to require a fair bit of change to the database before you can even start writing the code to handle it all.
What I understand is that each island is a separate database. Tom could delete Beluterra (for example) and it would have no affect on the other continents.
There's no burning need to do any linking, is there? Just create a religion with the same name, internal boards, rank structure, etc., and you're good to go.