I can definitely see how someone would think it's possible. I remember I used to have an even more major interest in breath play, and I thought it was totally possible to pass out from facesitting so I would keep trying to hold my breath until I passed out and realized I just couldn't do it and it was way to dangerous to push forward.

I'm sure it's possible but I imagine it's way too dangerous. Even despite my skepticism about it, I've seen fairly convincing videos (ESQ smotherqueenz comes to mind) that make me think it's possible, but even if it's possible I feel like messing around with actual ass(heh)phyxia is too dangerous to do even once, let alone increase the odds of something going wrong by making it a regular thing.
But i'm obviously no expert, just wanted to put my two cents in. I'm interested to see the other replies to this thread.