I wouldn't worry about it. I usually take rats out and throw them in hot water, then run some errands and come back a few to several hours later. If you happen to fall asleep and don't get to it until the next morning, I wouldnt be too concerned about it. just dont let it get excessive, periods of 20 hours or more I would start to have concerns, but if you defrost them in the afternoon and forget until the next morning, I think you're fine. I have never had any problems.
I just took a few rats out a few minutes ago, Im getting ready to go to a friends house to watch the Hockey playoffs, GO BLACKHAWKS!!!!!! I will not be home to feed until later tonight, should be no big deal. I think you will like the water defrosting method, saves a lot of time and will save you some money of wasting rats I think.