Heres Just a random thought
Random thought here ~
What about this.. its a bit of work .. for latex props that are pre painted.. (which would obviously include all props, animatronics, etc)
Rough up the surface ... and break the "latex barrier" of the prop. Without destroying the integrity of the item. Either with sand paper or a Dremel tool.
Then spray the item with fire proofing.... Would that not work ???? Would the FR not get deep enough ??
Obviously .. adding a FR to the paint or latex would be the best option and less labor intensive.. but just a thought.