Nope! Fiber is the wrong thing to throw in during a fast, which should remove the task of digestion from the body. Save the P&B for a cleansing diet post-fast! I have read some hairy posts from folks getting themselves good and plugged up with P&B while fasting.
The salt in the swf exits along with the water. I have read the chemical explanation, something about it being the same density as blood... but in general the entire quart, including salt, flushes out within 1-2 hours. It is an excellent top to bottom flush of the entire digestive system, icky as it seems! I have done MANY swfs, still don't like them, but LOVE the result.
A plain water enema is the best substitute. This can be done anytime at your convenience, and unlike the swf you have complete control!