Presume that if the hull ignites @ 540 F, then.....
I don't think the the hull 'ignites' at 540F! I am not a composites person but I think the 540F number has more to do with the degradation of the material on exposure to high temperatures and there are, indeed, specifications for how hot things can be if they are in contact with the 787 composite. This material (or to be more accurate the resin within it)
can self ignite given enough heat input but it takes a lot - around 31KW/sq metre with surface temps >500C or lower heat inputs if a flame is already present. See:
http://www.fire.tc.faa.gov/pdf/07-57.pdf for the FAA report on 787 composite flam tests.