If you can manage it, I'd give up the coffee for a little bit. It will help with the constipation & also should make the fentanyl last a bit longer (anything that raises body temp a bit will cause it to release faster -- hot showers, the flu, caffeine -- even in the tiny amount found in decaf, etc.). There are OTC products that can also be used to help with constipation (I was on Senakot -- it worked pretty well). OJ doesn't have a lot of water in it. If you can't stand water or herbal tea, basically any of the red or pink colored fruits (except grapefruit, which interacts with fentanyl) are good for hydration.
You'll be fine waiting until Monday as long as your hr isn't totally out of control & you're able to keep down fluids (which it sounds like you are). The Catapres (clonidine) patches can't be used until you're totally off the fentanyl anyways.
Fasting + fentanyl = dangerous. If you're going to fast, wait until you've been totally off the patches for 14 days. The drug metabolizes differently when fasting/dehydrated. Trust me, having to be taken to the infusion unit for fluids and nutrition sucks way more than whatever you're going through right now. When I had to go, there were even people who were at least 50 pounds overweight who had to go. Cutting back on calories is fine, but you still need at least 1000 calories per day, unless your PM has said otherwise.