I too have been considering this setup. I have five shamans at 70 and a paladin that was single-boxed to 80 and is geared for tanking raids. I was going to substitute one of the shamans for the pally and respec another shaman to resto, and then drag the shamans through instances repeatedly to level them to 80 and gear them up along the way. However, it's been suggested that just having four elemental shamans casting staggered lesser healing waves on the tank as part of their DPS cycle would be perfectly adequate. Something like:

Shaman 1: LHW, LB, LB, LB
Shaman 2: LB, LHW, LB, LB
Shaman 3: LB, LB, LHW, LB
Shaman 4: LB, LB, LB, LHW

I don't know if this would be enough. Once I get around to testing it, I'll feed back.