There was thread on this some time ago (2 years?). As was mentioned, the combustion by products of gas contain water vapor, exactly what you are trying to keep away from the rods by heating them. The heat source is not restricted, the hydrogen control is; but the codes don't dictate how to accomplish that.

You could use a gas heater to heat up some kind of a box but you must keep the rods isolated from the exhaust. Giovanni Crisi had sent me an article where that was done for rod storage in a remote location. Basically, a box within a box was fabricated with the inner box used to store the rods in. The outer box was fitted with a gas burner, thermostat, air intake and exhaust vents.
As I remember, engine fuel to run the generators all night was not as easy to come by as natural gas was.

Some have tried using engine exhaust in the same way. It will work until the engine is shut off and then you need another heat source. And temperature control is hard to control, so that really is not the most practical way to heat an oven.

Chet Guilford