Going back to the original question, about pressure breathing manually. In other words the objective is to raise the air pressure in your lungs significantly higher than ambient (and thus helping the transfer of O2 to your blood) by blocking your throat somehow and using your rib cage and diaphragm to pressurize your lungs. Hold this for a few seconds, exhale and inhale quickly and hold again.
I have never tried this to increase ppO2, but I have been taught this, combined with applying muscle tension strategically, as part of the straining technique to combat high Gs in aerobatics. All I can tell is that it is extremely tiring, and very bad things may happen if you get it wrong. (G-LOC for instance.) In aerobatics you only need this for a few seconds at a time. For a prolonged high-altitude flight without oxygen/pressure suit/pressure hull I would be very, very careful.