[Arran]

Is this lamp in series or in parallel with a resistor? I know that it was very common in an early AC/DC set to connect the dial lamp in parallel with a dropping resistor that was in series with the filament string but since this is a Tropic which is an oddball set compared to other Philcos nothing would surprise me. I think your idea of using a 120 volt bayonet bulb for a dial lamp sounds fine, or a C7 candelabra lamp, no doubt that arrangement has something to do with it being a universal AC/DC power supply export model.
I don't know how much interference a straight 120 volt LED would make, the puck lights over my kitchen counter create some noise but I think it may be the power supplies causing it rather then the LEDs themselves. You can't hear anything on AM broadcast unless the radio is within a foot or two of them, but on one of the lower shortwave bands you can hear noise all over the room, very likely caused by the switchmode supplies radiating RF junk through the wiring.
Regards
Arran