The reaction between oxygen and unburned hydrocarbons generates heat; too much hydrocarbon causes the converter to melt down. Speaking theoretically only, I wonder if it would be possible to heat up the converter to operating temperature under an inert atmosphere like argon, and carefully control the oxygen levels so that the hydrocarbon would burn off slowly, keeping the heat build-up under control. It could probably be done in a lab.
On the other hand, one might simply run compressed air through the converter to evaporate the unburned gasoline.
On the other other hand, whatever unburned gas got dumped into the cat was probably burned up right away. Whatever damage there is has already been done, assuming the car is not driven again under the misfires are fixed.
Theory notwithstanding, a commercial, mechanic-provided "converter cleaning" sounds like a hoax until proven otherwise.