I'm a biologist, and many people I know have worked in labs that euthanized mice/rats using CO2 chambers. Everyone I've ever talked to says the same thing you did about CO2 chambers not being very humane.
Except when my BP doesn't get a good hold on the feeder, I believe that letting her constrict them live is probably the quickest death even if it's a violent one. My biggest problem with whacking the rat against something is that I imagine the chance of messing up and just causing a lot of pain is much greater than with cervical dislocation (the phrase I meant to use in my first post instead of neck-snapping) or CO2 chambers.