Winco,
To add to enginesuck's answer, yes, fuel has been known to migrate along the underside of the wings or the fuselage, but only the fuel streaming on the wing skin underneath the jet pipes will flow off into the jet efflux. Fuel on the outside of the fuselage originating from the bomb bay would have to travel upwards and outwards to reach the underside of the jet pipes. This would have to occur within the very thin boundary layer of air under the relative airflow, which means that the amount of fuel in that layer would have to be very small and go against gravity to get up under the jet pipes. Any large leak would be caught in the RAF and disappear.
I have heard of fuel lightly spraying into the jet efflux due to fuel leaks. None of those incidents resulted in a fire in the jet efflux.
The F-111 uses its reheat system to start the fire, which naturally projects rearwards into the RAF. When the pilot selects DUMP, he simply adds fuel to an existing fire.
Hope this helps
Ed Sett