If I was forced to put in the swing, I would drill 2 holes through the ceiling and into the roof. I would then lay down a 5/8 inch plywood up on the roof with the size to be about 18" beyond the 2 holes in all directions. So if the 2 holes are 3' apart the plywood would be about 3' wide by 6'. Would want to make sure there is a nice flat area up on the roof for that. Glue the plywood down and install 3/8" eye bolts with some good size washers up top. Paint and seal everything up on the roof.
Then make sure no body over 125 lbs uses it. And when using it, make sure you don't pull or yank on it to increase the stress on the bolts.
