@Abb, I think sacred shield actually out-does PW:S per 'cooldown' (weakened soul). So PW:S already has competition for being an awesome tank mechanic.
It would also continue to be unique, being a big fat frontloaded bubble compared to the steady ticking nature of sacred shield and this renew concept.
Since if you keep grace on an offtank, and roll renew on him indefinitely you'd have a whack-crack-omg bubble, I think it makes more sense to have it work ONLY on effective healing done by renew. Maybe have it be 30% or 40% of each tick.
This also means that the renew ticks are most effective (providing absorbs) when they're most needed (the tank has a health deficit).
Really cool idea though. I could see that making glyph of renew a pretty awesome tank healing glyph for disc.