Since the DRL's are actually the high beams at a reduced intensity, I doubt that you'll be able to come up with a lamp that is actually white at the reduced wattage. Xenon is definately a no-no for several reasons. Xenons use a ballast (transformer) to bump the voltage way beyond 12 volts. Additionally, they start-up at an even higher voltage then reduce down somewhat. At a reduced intensity, I bet that they'd extinguish themselves.
Xenons use an electric arc as a light source (Voltage drops, no more arc). Halogens and conventional lamps use a glowing filament as a light source so they can be regulated down. Look at light dimmers at the hardware store (for your home) and you'll see on the package that they are not intended for flourescent lamps...A little different, but the same principle applies.
Sorry to offer up bad news.
PS If you were willing to disassemble the lamp and risk destroying an expensive part, you might be able to get the effect you want by using high intensity white LEDs. It'd be a GIGANTIC high stakes gamble though...