So you will be full. You might be running to pee a lot, but hey, all the time you spend peeing will be less time at the table, right? At the party have water in a wine glass so you will have something to hold. And you can do the food pushing thing... no one will notice. Or if you are comfortable, just tell about your cleanse. I prefer to call it a cleanse and not a fast, people are more receptive/less judgemental that way.
You can do it... consider it one more challenge to get through.