I noticed this while doing general Vezax as holy (normally protection.)
When you dual-spec to holy, anyone with GREATER blessing of sanctuary has it removed. And you have blessing of sanctuary removed from yourself.
Everyone else in the raid keeps a normal blessing of sanctuary.
This means you can dual-spec prot, buff the raid with sanc, then dual-spec holy, drink+divineplea and then go pull the boss with the entire raid (other than you) taking 3% less damage.
It is a bit of an annoying effort to buff 24 people with sanc, but with PallyPower you can just hammer your auto-buff key 24 times which isn't too bad.