I don't know about offsite churches but I do know that the Polynesian hosts 2 masses on Sunday morning. I believe the times are 8:00 and 10:30. You can verify the times by calling the resort. If you aren't offended by people showing up in park clothes, it's really very nice. If you plan far enough in advance, you can make breakfast/brunch reservations at the resort or just grab transportation to your park destination. One tip, if you decide to go, try to arrive a little early if you want to sit in the shade. Mass is held in the dinner show facility and only the back portion of it is covered.