Is that a very common thing to do? I've never heard of that, but my nitrous experience is all with carbs. Doesn't sound like a very good idea to me. Why rely on the slower cooling of the air and sensor when you can just get much quicker, more reliable and more adjustable reaction electrically at the terminals for the same sensor? And if the sensor reacted to just the increased density due to the cold, well it really is that cold and should richen that much just for the cold but should richen even more for the added oxygen. So it would compensate for the cooling due to the expanson and state change etc., correcting for the cold, but not for the fact the oxygen concentration is so much more than air? Sounds like it would make the mix right for air injection, not nitrous injection. And the cold intake is a very good thing, why let it warm up in the airbox or take longer for the cold to get to the TBs? Bad enough that most ECUs take a delayed time-average of the air pressure pulses of a ram-air system, especially in a big-twin with big vacuum pulses, so they take a second to react. But I guess maybe the slight delay with the nitrous coming in so far upstream, maybe the timing for the delayed nitrous and the delayed richening work out OK? Does the hit come on smoother?
Of course the nitrous won't burn (DOH!) but plastic sure will, and the high oxygen content present will make almost anything burn like mad, including the plastic of your airbox. More than one bike has burnt to the ground 'cause of too much nitrous. Thick steel will sustain fire in pure oxygen. I think aluminum will too. Once you start cutting thick steel with an oxy-acetylene torch, you can then turn off the acetylene completely and keep right on cutting with just the oxygen stream, 'cause the steel itself will burn with just the oxygen stream once it gets started. Same thing with your engine parts, too much oxygen and the pistons and valves don't just get burned, they literally will burn up. 'course the nitrous isn't pure oxygen, just has more than air does. I'd think the polypropolene of your airbox might burn in a flash like it was made of paper if it's filled with nitrous instead of air. Sure looked like that to me when a guy hit the button at the wrong time without enough revs or the throttle open enough. Then again, he could have been running a wet shot and spraying fuel & nitrous for all I know.