If it were me I think I would try and stay in the Serenity state as long as I could, using Holy Word: Serenity as soon as its off cooldown. This will keep renew ticking without costing mana and increasing the chance to crit with heals.
I'd spec into Tome of Light and State of Mind. This would let me use HW: Serenity more often and even though State of Mind only procs from heal (being changed in 4.0.6) the extra 4 seconds per heal would be useful. I might also consider speccing a couple of points out of Darkness and into Veiled Shadows so I could get a couple of fiends in one fight.
As someone else suggested reforge some mastery / crit into haste to give you greater throughput. (Especially if you take points out of Veiled Shadows)
I'm not saying you must do this or that it will grantee success as I understand this probably isn't optimal but if you are having to tank heal and no one else can you just have to try different things out and see if one works.