Yes it is possible and I have done it, but I am very flexible and practiced both yoga and martial arts for a decent amount of time. basically, if you're sitting and leaning back with legs extended, and your bottom sits on you facing you, his legs sort scissoring your torso and maybe he's bent at the knees a little, if you bend down you can get his dick in your mouth as he bounces on your dick. I did this a couple of times with a nearly equally flexible partner -- and he wasn't hung at all, we did this with flexibility alone. I was proud that we pulled it off and it was a fun experimental thing to do. And it felt great to have his dick in my mouth while I was fucking him.