don't know if this is as humane, but you can also put them in a very small airtight tupperware. the smaller the bowl and the more mice/rats you can stick in there the better. you don't have to use any co2, just set the tiny tupperware near the heat lamps so their bodies don't get cold. it has to be AIR TIGHT. shake the bowl to make sure they are out and leave it for like 1 or 2 minutes longer. i've had them wake up on me before too, it's pretty pitiful. the tupperwares i use are only as big as the mouse. the smaller the bowl, the less they have to suffer.

i've also heard of using airduster, the stuff you use to clean electronics with, human teens get high off it all the time (so i figure it's less pain for the rats), i would use a tiny tupperware like stated above but put a hole only big enough for the straw, squeeze the trigger for like one second and plug the hole. if you're going to use your finger to plug the hole i would put some saran wrap over it for protection from scratching.