Yeah, been lucking enough to experience that a couple of times, and a P-slide a few times. Feels AWESOME, but on the other hand, was one of my dumber moments of weakness in this hobby. It's pretty much as close to bareback sex as you can get depending on how deep and long you're rubbing through her crack, and at that point there's no barrier between your skin/secretions and hers...
That combination makes for the highest risk I could conceive of at an MPA. Heck even a BBBJ by the MP would be "safer" (in my opinion) since it's oral-genital contact rather than genital-genital contact... But that's just my opinion, and I'm not an M.D., just another faithful pervert.
Good luck to those searching for it, I'd make recommendations, but both of my favorite girls are out of the biz.