This may not go live, but on the 4.2 PTR, Holy Paladin Mastery shields are now stacking up to a maximum of 1/3 of the Holy Paladin's maximum health.
This is a pretty good change, in my view, and will probably be enough to bring Mastery up to a viable level.
Of course, there are still some concerns, specifically with Beacon of Light. BoL heals don't create shields, so your BoL target does not benefit from your mastery. It's perfectly fine for 2-tank situations, but Mastery does become a little weaker in 1-tank situations or if you prefer to heal your Beacon target for Tower of Radiance power gains.
And of course, it does encourage paladins to heal a tank who is already at full health, to build the shield higher. But on the whole, this should be a good change.