I can see how flying through a cloud of your (or someone else's) fuel might not be a great idea - mostly the smell and smeary windscreens, I'd think - but what's the worst that could happen if you dumped near a thunderstorm, low level with the radar on while trying to call Mumbai on HF? You're not going to have flames chasing back up into the wing, like in cartoons. Mythbusters tried it with petrol and were down to almost walking pace before the flames caught up and even then, nothing happened.
F111's used to deliberately "dump and burn" for airshows but they had to have the reheat on to ignite the fuel,
AFAIK.