For more than 30 years Petzl has led the way in as an innovator in the research, design and manufacture of quality headtorches. From humble beginnings, when Fernand Petzl fashioned the first helmet-mounted light for caving, headtorches were refined over time, until he created the 'Zoom' model, which, with its rotating bezel as a switch and focus adjuster, became a seminal outdoor product well into the 21st century.
Come on, move with the times. All you need to do to boost lighting in extremis when benighted mid-pitch on a long route is take one hand off the tiny crimp you're hanging on, fish your phone out of your pocket (you didn't leave it in the top pocket of your sack did you?), and use the convenient app to adjust the brightness.
Problem is battery - either the light has to be sufficiently crap that the battery will last for a long time, or you have to be able to use lower brightnesses to avoid using up all of the battery in an hour or so. So in answer to your questions, probably the really low-brightness ones?
They couldn't just put a little LCD battery indicator on it or some traffic light LED's? I have a MTB handlebar headlamp that tells you how many hours of light you have left and on what brightness setting with the use of a single RGB LED.
Seems to me that fitting Bluetooth to a head torch is like fitting suspension springs on a library card, costly and completely pointless.