I've subsequently used it on furniture that was dented when moving house.
On the 'does it work on a sealed finish' question, the first move is to try. If it doesn't work, there is a further option for this technique. When I bought my used Rob Allen, there was a ding in the the neck, which is finished in nitro. Rather than just ahead on an expensive instrument, I contacted Rob Allen and he said that if the finish was unbroken, a small score with a razor blade would open up the finish for the steam treatment. It turns out that the finish was broken anyway and the dent lifted and is still fine to this day.