I would first try an experiment with plain water: Boil some water as you suggested and mix it with cold to, say, two degrees warmer than your preferred developing temperature. Then agitate the plain water from time to time as if it were developing film, until the time when you would normally stop development, and check its temperature then.
If it has cooled down four degrees in that time, the same starting temperature with developer should work just fine (the "normal" deve temp will be reached at about half way through). On the other hand, if it has warmed up, or cooled differently, you can calculate a better starting temperature that will average out to your normal one.
This works best if your darkroom air temperature is pretty close to the developer temperature.
Good luck, and let us know how this works.