I'm looking to mess around with a team for the role of support in battlegrounds.
I doubt this team would participate in arena or rated battlegrounds, as a team.
Probably run them 1-boxed for conquest points, each week.

From the point of view of normal battlegrounds.
Primarily the larger battlegrounds like: AV, IoC or SotA.

In your opinion, which set of 5x Healers would have the most effective offensive pressure?
I'm fairly sure, playing support and sticking with the largest group of players, any set of 5x Healers can heal reasonably well.
For simplicity, I'd be sticking with five of the same class.

a) Disc Priest
b) Holy Priest
c) Resto Druid
d) Shockadin Pally
e) Resto Shaman