Two ways for this to work. One, the flat rate shield. Where every inner beast gives so much damage reduction upon use, which is exactly what SE is doing. Two the excess hp shield. Where any over heal from an inner beast becomes that shield, so that you use inner beast at full health and if it would give you 1k hp, it gives you a stone skin for 1k. That becomes useless pretty fast. First, you will never be sitting with banked hp for long, and in harder hitting fights it may never happen. It scales very poorly with progression, as in it will only scale when you improve not when your enemies do. Second, you will have to be 100% synchronized with your healer. You use IB too soon and suddenly a heal is wasted on top of not having that extra shield. Healers would also have to have rough ideas of how much of that shield you have banked at any given time to know how much to be ready to heal you for.

In short I just don't think it would work very well, and the given changes are by far better.