I like the OP's idea. Its better than weird, unmotivated and uninspired tacking-on of a surecast effect to random buffs a PLD has, and actually works to tie together two otherwise distinct, separate, and unrelated mechanics/abilities. You could literally just Sheltron+Clemency every physical TB though, providing a mere block be enough to keep you from dying, which would be pretty OP. But I like the idea of tying mechanics and abilities together to make the job feel more in sync with itself.