I repaired a steam cleaner that may work. Seems to remove stubborn stuff and leave the surface dry.
Traditionally, wooden barrels used for fermenting beer were 'scolded' then air dried in the wind. So boiling water worked in the past on absorbent wooden barrels, but this may be because the wood trapped more of the good yeast than wild yeasts so when started again (usually straight away) good yeast had the advantage. Not sure what happened when you started making infected beer though. Maybe put it over a fire - that would steam it for sure!
Doesn't work for modern plastics though
For mini kegs they should be ok in the oven without any plastic bits attached. Don't get too hot as there may be a lacquer on the metal.
You could boil them in a pan with a bit of hot water inside so it all gets up to steam temperature. I think the issue is keeping the heat for long enough. I sterilise my jam jars in a pan of boiling water and seems to work filled straight away with hot jam. Forms a vacuum seal when cold and has kept jam for 2 years+ like that.