its a good questions.. i dont think powdered fiber would help..
think about things that would absorb the protein shake in.. like for example pwo i like a white bagel with my shake.. well if you know anything about bagels, when they get wet the absorb the water.. same thing with a shake.. it would slow down the digestion of the shake, which means more absorption.. not by much, but still more.. liquids pass through your system fast.. that's just how your made..
this is all hypothetical btw.. just by exp and possibility