IGH,
Interesting, but from your quoted text:-
Ops Manual mandated a "clean" configuration for fuel dump; tests showed no significant hazard in any configuration Lockheed next tested ignition of dumped-fuel: test Tunnel, scale model wing and dump system ==> test proved it was impossible to involve the Wing in fire under any condition (unless a fire was maintained within the Wing itself.)
Seems like under idealised conditions, you MIGHT be able to ignite the vapour trail, but even then it is unlikely to pose a hazard.
Still, that is more what I was looking for in terms of SOME type of evidence, rather than just "What I heard" type reasoning.