^But inductive AC ripple wouldn't create 115VDC potential to earth, would it? Wouldn't it be just.. plus and minus xV from earth (or whatever you were measuring it against?) The capacitor would block DC.
This iron actually has an earthing screw on top above the power connector.
This makes sense to me (but maybe I'm an idiot). So perhaps the earthing screw is connected to a wire in the cable? Or to a shield in the cable which eventually runs to the sheath of the T12 style tip? In this case, you could snip the connection and install a switch to select grounded vs floating tip?
Also, since it runs off DC, perhaps the designers assume that the DC ground will be grounded to earth IF/WHEN the unit is run off an actual grounded smps? So perhaps even in the hand piece itself, the 3 connections of the tip, 2 of them are shorted together? So it might even require modification to the iron internals, itself, to liberate the sheath from any connection?
Or if the fit of the tip in the handpiece is loose enough, you could maybe wrap the base of the sheath/tip in kapton tape, so only the 2 heater/thermocouple connections are exposed? Jam that back into the handpiece, and shazam, you have an isolated tip?
I don't have T12 iron, but I would assume if you equate it with a TRS stereo plug, the base ring is the tip/earth and the middle and tip rings are to the heater/thermocouple unit.