If we work backwards, it seems that the ideal is to maintain a short stretch of fuel line, occurring before the control valve, at a given temperature, the temperature required to vaporize the fuel traveling through the line. Once the stove is going, the heat available to do this is the cooking flame, and Roger has tried various conductive paths-- extra metal of various types, the metal of the stove itself--to achieve this. What about a more active conductive path, one that was self-regulating?
Just as a thought experiment, imagine that we could use the heat of the stove to generate electricity, then use that electricity to heat the tube, with an intervening active thermostat-like circuit that would maintain the tubing temp at the correct point-- we'd be done. Environmental factors, wind screens, would all be taken care of.
Not practical yet. But perhaps there's some kind of thermocouple-like solution possible, one that would open or close a heat path as needed?