Having spent many years constipated I understand your worries. However, I've found that at the times when my bowels actually got working regularly (and I still struggle with that)the experience was totally different than I was used to. When you are going 2 or three times a day it is real easy. It's no longer a major production. It takes only slightly longer than peeing, and there is usually not a splash either because it is softer, usually comes out on one or two long guys that slide right into the water.
The problem becomes that you can get constipated by the P+B itself. That happens from not enough water intake, or not enough water immediately chasing the P+B shake. Then you'd have the problems you're suggesting.
Flax seeds are great for regularity. I take a couple tablespoons and put them alone into a very dry blender and blend till they are fine. Then you can add other ingredients. I always put in a bannana, and then juice or rice milk and then I add frozen berries, sometimes a little honey or organic Sugar if it needs it. It makes a yummy shake. And really helps ya get those nice easy poops!
I've found that with P+B you have to work your way up in dosage according to your body's response. If you aren't getting one or two bowel movements a day then decrease the dose, until you are. Then in a few days try up-ing the dose again.