USB-C has a lot of pins, some custom built cable could use nonstandard pins for power and data and the standard data ones for the 20kV line.
Or just run the 20kV line on the data pins and leave power standard. That way you can use a power-only cable to charge the phone.
True, didn't think of that.
I guess a much lower voltage of let's say 200 volts would kill a device just as effectively and probably wouldn't arc itself.
Another option would be a smart device that first senses some current and only then dumps the 20kV.