I don't think people got the point of your question but I think I know what you might be getting at.
You're water fasting, and want to keep digestion going to get rid of toxins, so perhaps you might be thinking that p&b shakes will help keep things moving, right?
I'll tell you my experience with water fasting...my digestion literally comes to a standstill. At least with The Master Cleanse I'd get bm's everyday. With water fasting, none. I'd be scared to do the shakes because of my fear of a 'backup' in the colon. I wish I could be of more help.