Are the headlights HID or normal incandescent in a projector housing?
Either way, find the wire that controls the projector high beam cutoff shield, and put the flasher to that wire, giving it +12v will activate the high beam by lifting the shutter. I'm not sure why no one flashes high beams with bi-xenon projectors, I don't see why it wouldn't work.
Now of course you'd also have to give +12v to the lamp itself for when the headlights aren't on.