You might be overthinking this one. I believe you will be able to extract the mandrel with a few taps, especially if you beeswax it up first.
If it proves impossible even after a little gentle heating for expansion, you can always drill the crown, it needs a hole to bolt onto the kingpin top anyway, tap it for air or grease fitting, epoxy up the air hole in your mandrel, then blow it out with a grease gun or airline.