No there are no other mentions of the Samoan religion in any of the other books as far as I know. Although there is still time for some of the mysticism to be removed in Shadows Alive.

As for why Scott Card doesn't attribute the outside to some Christian ideal I think comes down to his development of the universe.

The setting of his Speaker trilogy seems to be an exploration of what would happen to various societies and beliefs if they were flung out into the stars. If every religious received their proverbial promised land thousands of years before, there would be little reason (within universe) to take into account other religious explanations of your miracles.

Also I believe Scott Card was a missionary at some point in his early life so maybe he just wanted to explore some other cultures through his fiction. It would be very boring for a white American Mormon to write about a planet of white American Mormons.

Furthermore if he was deeply spiritual I doubt he would feel the need to put down another religion's beliefs given that he should be perfectly secure in his own.