It would definitely be fun at first, but overall I think it's going to be another gimmick team.

Though I haven't done 5 priests, my original time was 2x affliction warlock and 2x shadow priest. On paper, 12-15 dots, most instant cast, sounded absolutely amazing. In the real world, we had several issues:

1. Multiboxers don't have true mobility required of most cloth classes (especially mages). Melee will rip you in half.
2. Millions of dots != burst damage. I'd watch geared druids heal and HoT right through 12 dots + 4x lasers. Mind blast + shadow word death is nice, but long cooldowns mean you're probably only going to nuke one person before your group gets wiped
3. Lack of healing (unless you just want a bunch of disc priests) makes it hard to survive, especially for your main. Again : cloth + stationary = dead

These reasons, and more, are why quad shaman are the typical pvp setup. Reasons are:
1. Only stationary ranged caster. heavy armor + totems = you don't really need to kite, but you can if you want. And with magma totems hitting so far, you probably don't want to kite - these days i let magma and fire nova totems eat through the melee while I nuke ranged 1 by 1
2. Totems that enhance the party - fear breaks,
3. Able to heal if things get nasty

All that being said, it would still be fun. Viable? As in, actually being as good or better than 5 individual players and what they'd bring to the BG? Doubtful. Even quad ele shaman, before they get a ton of gear and some skill, generally hurt your team in BGs because 4 players can just accomplish more (especially when those four players are likely DKs and ret paladins, or huntards with explosive shot and 5000 attack power).

On the other hand, I'm very intrigued by a 5 disc priest setup. Not even to nuke, just to annoy people to death. 5x prayer of mending, aoe heals, all kinds of stuff. I supposed you could do the same with shaman (I'm debating respeccing them all resto for a few days and just chain healing and riptiding pepole to death ... that or healing the entier bg myself