I don't like the idea of installing an open back fixture on combustible material even if there is a proper J box behind it. It still allows the wiring in proximity to the combustible material. I bet if you reasearch hard enough you may find that these fixtures are to be installed on non combustible wall or ceiling finishes
You got it, Non combustible does not include wood
Using the pendent with a pancake box for wiring space is fine provided the mounting area is non combustible
When you see them on wood it is usually handy dandy electric
The k&T wiring connections were on plaster and they made a hole and wired in the canopy area
I hope reads ok I am on the move typing on the phone