I'm pretty flexible, and when I was a teen, I could do it.
I remember being surprised at how hot my glans was. I could only get about halfway down my shaft with my lips (not to the base), and although sucking along that length felt good, my favorite thing to do was to lick the opening. I never came that way (I could only hold the position for a short time), but it was nice.
Haven't tried that in years, but I remember a few years back, getting in a similar position in yoga class and being flexible enough to put both feet behind my head while lying on my back...which, by the way, really brings out the bulge in one's shorts. My female friends in the class applauded, one of them smiling while looking somewhere other than my feet, saying, "Lookin' good, Parker..." ?