I think you are worrying excessively about this (although it is good to worry a bit about it).
The wire carrying 300 volts should have insulation rated at 300 volts or more, so it can lie against a grounded chassis with no problems, so there should be no problem if it contacts the metal case of the switch. You should make sure that the metal parts of the switch are grounded by mounting the switch in a grounded metal panel, or running a wire from the metal parts to the circuit ground.
Good construction practice recommends that high voltage wiring should be routed away from low voltage digital wiring, and that all wiring should be secured with wire ties or cable clamps. This should eliminate the possiblility of your high voltage wiring from contacting the metal parts of the switch (or anything else the user might contact).
All switches I've used have had the user-accesible parts well insulated from the actual switch contacts.