The ratchet is my backup method & I'm really surprised more people don't use it (fast, ~painless, cheap, proven). It's gotta be more comfortable than hanging one's whole bodyweight by one's neck and certainly faster and ~likely more comfortable than SN (perhaps a lot more).
My plan/understanding with my ratchet is -- positioned properly high on the neck... above the "adams apple" area of course -- to keep pretty quickly -- keep-on cranking until I fainted, trying to push a few more clicks in at the end. While I can see how what you propose might be enjoyable... I think you need to know whether you're passing-out (fainting) due to the ratchet or due to the N2O = If you faint due to the whippits, you might not complete the ratcheting enough and that could lead to brain-damaged survival (particularly without a bag... a bag is generally is not required with the ratchet*, but may be used if desired).
HOWEVER, you plan to use a bag. This adds the complication of getting both the bag and the ratchet properly positioned on your neck... at the same time. AND, with managing the nitrous oxide N2O from leaking from the bag. I'm not sure you have enough N2O to come even close to filling a bag = if the NO2 got pretty diluted, will it have any effect?
FINALLY, >>> and please others comment on this as this is my speculation <<<: Typically, with the ratchet you're breathing regular, room air*. If your bag is small, carbon dioxide (CO2) will start to really build-up. If you're already passed-out, maybe no problem. But, CO2, not oxygen, is more what your body senses when it feels it's suffocating. If the CO2 builds-up in the bag and somehow you become aware of it, it may be unpleasant.
That's my thinking from the best of my knowledge...
P.S. If you follow the sensations as you pass out from the ratchet alone... you may find them just as pleasant as whippits (albeit likely somewhat different feeling).
* According to the instructions I follow from the "Five Last Acts" book by Docker.