This means if you're sunderspamming, you can't queue up shield block
Shield block ignores the global cooldown.
Also, comparing the threat/rage of shield slam to revenge is practically meaningless when comparing shield slam to sunder armor. Personally, I use shield slam almost every time it's up, second only to revenge.
But back on topic: I would love to see this mod as well; possibly using CandyBar to indicate duration remaining, or even just the default clock-style cooldown used on action buttons.