High beams aren't good candidates for HID conversions. For one, the bulbs take a few moments to fire up and stabilize, so flashing your lights wouldn't work well and could possibly shorten the lifespan of the ballast and/or bulb.
New BMWs have dual-mode xenons that re-aim the low-beams in a high-beam pattern if they are kept on, but halogen lamps are still retained for momentary operation.
