Trust me, I have some of the 'smart mass,' and you wouldn't want to try it... Even in a soft grip, it would flex and move around, slowly squeezing out and going where you don't want it to be. Also, it can be a little adherent to plastics, and I suspect that it might, when pulled from the body, affect the finish of the sides of the phone.
The shock absorption is really impressive. I don't know how well such a thin layer of it would translate when sealed within a phone casing, but I have no doubt it would be much more effective than a standard TPU case.