ORIGINAL: DocYates
The key will be the oxygen. Not air, but oxygen. The vaporized fuel in the tailstream of the turbie is a rich environment for flame. the particles are dispersed so finely that the refractive index of the moelcules is giving you the illusion of a white vapor. If you could ''energize'' that vapor with a little oxygen (higher concentration than the 14% in normal atmosphere) you would have a rapidly burning mixture (when ignited) that would carbonize and leave you with a thick heavy trail of black smoke, just like the F4. The key would be two things. First a one way valve on the oxygen line to prevent burn back up the line, and second some form of ignition once the vaporized fuel / O2 mix is in the stream. you would also need a fair amount of protection on the rear of the plane, it is gonna get real hot back there.
Believe me, I have seen things burn with oxygen that you would never think could catch fire. All it will take is a small amount in the fuel vapor.
I don't think introducing pure O2 would be the right thing to do. The combustion would be much more efficient than with atmospheric O2 likely producing less Carbon and more clean combustion products (Co2 H20 etc..)
I would research more along the lines of either suspending a carbon product (creating an aerosol/suspended particles) EG the "white" smoke oil mechanism. Or obtaining a less effiecient combustion.. Just my HO.
Someone mentioned Carbon black which would work, but dispersing it into fine droplets is another thing..
Pete