I only really have 2 things to add. By priming the container with Co2 (adding the dry ice before the prey), you will decrease the amount of time from 2 minutes to 10-20 seconds for them to fall asleep.
My only reservation with doing that is because with this method, I have no clue how high of a concentration of CO2 will actually be in the chamber when the rats are introduced. Exposing them to even a moderate level of CO2 can cause respitory distress and undermine the whole "humane" thing we're going for here. Speeding up the process with CO2 in bubbling water may further increase the likelihood of "overpriming" the chamber.

To me it just makes more sense to start at zero and gradually increase from there, reducing any margin of error. It surely doesn't hurt them at all to be exposed to a gradual increase in CO2, even if it means being in the chamber for an extra minute or two.

I think you'd run into a problem with scale when you start talking about euthanizing larger animals, though. The page I referenced said that this method wouldn't be appropriate for animals over 8-10lbs.