Alright: the kicker (and the reason why I don't think this will result in a black hole) is that in order to extract that energy from the particle, you have to c

Alright: the kicker (and the reason why I don't think this will result in a black hole) is that in order to extract that energy from the particle, you have to continuously re-heat it and add more energy back each time you want kT \ln 2 units of work done. Remember that the work is done isothermally, so the particle's energy is actually constant throughout this process. What's really happening is that if we consider the positional information to be continuous, then the particle's positional entropy is basically infinite. We can reduce this entropy (locally, to the particle) with a measurement, and use that lowered entropy to extract as much isothermal work as we want. But its energy density is unaffected by the positional information.