For someone with a lot of money and support or strong legs to carry a spare battery, the 1200k brevet solution is a Lupine Sl AX or Supernova B54 mini, both of which have a dipped StVZO low beam but it also has a high beam at the push of a button. So, you can read signs or avoid hitting a curb when cornering.
A strong helmet light powered on in a pack ain't right. On a 400k this year, there was over 300 turns on the cuesheet. Turning a headlight on and off seems problematic but I don't know. That ride was my first brevet with a StVZO beam and possibly my last.