It sounds like the load resistance needs to make itself 15/40 ohm for the lowest case and 30/40 ohm for the higher case.
I'm recalling how incandescent lamps vary their own resistance. At least the old-style incandescents do. Resistance goes up as the filament glows.
An automotive headlight draws around 3A at 12V. That makes a 4 ohm load at 12V. At 6v it might present 2 or 3 ohms. This is just a guess.
One headlight won't stand up to 30V. But suppose you were to put two headlamps in series. You'd need several such strings to bring the total load down below one ohm.
Anyway the right combination of lamps and shunt resistors might suit your requirements.