Down the road, it will be very easy.
Since a Greater Blessing hits every class in the party/raid of that type, have each Pally do a different Greater Blessing.
They can /cast [target=player] very easily, with each one targeting themself and giving the buff to all of the pallies.

In the meantime (ie. until you get all your Greater Blessings), or if you prefer an addon solution check out Pally Power.
You can have each Pally have their buff keybound to the same key.
The group/raid leader (or anyone with an assist in a raid) can choose which pally casts which buff on each target.