You are correct that the charging circuit requires the GEN bulb filament to complete the charging circuit. Knowing that bulbs do not last forever, our Panteras use a crude but effective add-on to provide a charging circuit path if the bulb burns out.
A ceramic resistor was soldered across the two terminals on the lightbulb socket.
stock resistor is 15 ohms rated at 10 W. If this needs to be replaced the same 15 ohm resistance should be retained, but wattage is not critical as long as it’s no less than 10 W
Larry