If I recall correctly, the old Stanley steamers had a hand pump on them to pump up fuel pressure for cold starts, similar to pumping up the pressure on a Coleman camp stove. Maybe a pressurized fuel tank could be set up to provide enough pressure to push the fuel through the nozzle and atomize it at least until steam pressure is built up.
That's an interesting idea but this boiler has an "air" powered atomizer. The oil is gravity fed to the center nozzle and "air" is blown past in a venturi fashion drawing out the fuel and spraying it in to the firebox of the boiler. So there is no way to pressurize the fuel to spray it. Using the propane in what I call a "limp flame" mode just burning out of the atomizer may generate enough heat to boil the water and build about 10 pounds of steam pressure to switch over to steam atomization of the diesel. All good idea's but so far I'm basically down to the Initial atomization of the diesel with nitrogen which works or the propane that I still have to try.
Keep the idea's coming.