After having a couple of stomachs perforate during cleaning (what a wretched smell!), I tried the fasting thing. Yes, they had very little in their gut, but I just couldn't do it again. I tried, but they would come over to me eagerly when I came in to feed everybunny else. They just had these "SO happy to see you, we're hungry, and it's time to eat! YAY!!" expressions on their faces and all over their bodies.
So I started giving them only hay to eat the last couple of days. Had a stomach perforate, and it hardly smelled at all! So now that's what I do. If something comes up preventing freezer camp, I can just put food back in again until the next weekend.
I read about withholding water, but I could not even consider that. A full bladder may be something to be worked around carefully, but it's not that hard.