In a gas motor, that would be diagnostic of valve seals, because the motor draws a vacuum when the throttle is closed. In the diesel, you are on the right track with your ideas. When you are on the throttle, you are burning diesel injected through the injectors, and any oil that makes it past the o-rings. When you let off the throttle, the injectors stop firing while you are decelerating (on my truck, until the torque converter unlocks) - at which point you would be burning only the oil passing the o-rings.
However - because you say huge clouds of smoke - I'd be looking at the turbo first.