Can oxygen diffuse out through the lungs? Let's say you accidentally walk into a room pressurized with pure nitrogen (or you're jettisoned into space). Within a couple of seconds, the partial pressure of oxygen within your lungs drops to 0. But there's still a (small?) reserve within your bloodstream. Now, will the oxygenated blood travel to your lungs? If so, will the usual gas exchange reverse, with oxygen going _from_ your blood _to_ the oxygen-poor environment?

Yes. Blood will continue to circulate (as long as you're alive and your heart is beating), so will eventually come in contact with your lungs. Once there, the gasses (O2, CO2) will cross out into the lung and be exhaled. All diffusion is driven down a concentration gradient.