I have been doing khechari mudra for quite some time and have indeed explored the nasal pharynx, septum and entire space, inside out.
I just stumbled on this article online that said it is possible to extend your tongue all the way to the eyebrows. But I just don't see how that would be possible. My tongue is long enough and I pretty much hit the roof beyond which there is no gap or hole to reach the space behind eyebrows.
So is it just figurative when they say you must take your tongue behind the eyebrows or is there a way to really do that?
By angling your tongue by about 45 degrees you can make the tip of the tongue go up the nasal passages, have you tried that?
How I do it is to hit the roof that is above the top of the septum, move my tongue to the left or right to get to a nasal passage, push forward a bit toward the nasal passage and I hit some grooves that help me turn my tongue. I find that it can then help to plug the opposite nostril, then inhale powerfully to create a bit of suction to help pull the tongue further into the nasal passage.
When I do this I get to what feels like it is somewhere pretty close to being at the same level as between the eyebrows.