I wouldn't say it's impossible. The mapping process has to be done once, set an anchor and then you shouldn't have to stumble around blindly anymore. Honestly though, you can just rest the headset on top of your head, just use the haptic feedback to mark corners. Maybe i'll tweak haptic values to give a better sense of what action you're doing, but as long as the headset is following you and tracking the controller positions, its possible to do setup without looking in VR.
Its weird that Oculus has gone as far as to disable passthrough visuals entirely while the guardian is off. That double-tap shortcut would be a big assist. They have mentioned the possibility of opening it up in the future to devs like me, and that will allow me to make the setup into what it 'should' be.
You know how the guardian system can already do the 'show red dots when it detects an object might be in your playspace'? Well, that's the stuff right there. You could just build the basic geometry of a space by walking around in it. Its all there, just locked away from me. Kinda frustrating. Buuuuut, like i said, its worth it to stumble through the setup, as is. You can adjust wall positions, add in new areas later on, start with a small space, one room or two, and then find an anchor that works.. build out from there in later sessions.