One druid (1500~healing) should be able to maintain 3x 600~HPS for almost any length of fight, you could maybe squeeze in a 4th, or have Rejuv in there too:
MT - 600[Lifebloom] = 600HPS
OT1 - 600[Lifebloom] = 600HPS
OT2 - 600[Lifebloom]
[top] 600HPS
Total
1800HPS over 3 targets
It'll never allow for the 5sec rule regen but with a decent MP5 setup (gear/spec) and using appropriate consumables with Innervate too, its endurable.
While paladins may still be able to maintain higher HPS on one target, druids will be able to pull out ahead of them on encounters that require more than one tank.
This isnt even including the cheeze with sustained trinket-Lifeblooms, which would put it somewhat above 700HPS on the offtanks, and 930HPS on the MT (=2330HPS).
Note: I highly doubt a tree druid could sustain this as long as a dreamstate druid could.
[Estimated Math, not 101% accurate but close enough for general speculation]
Using Dreamstate spec:
2400MPM [200MP5 Raid buffs/gear]
600MPM [50MP5 BOW]
300MPM [25MP5 Flask]
1200MPM [Mana Pots]
=4500 Mana Per Min (No Innervate, no shaman totem, no SP)
Rounded up:
1900MPM [LB]
2100MPM [Rejuv]
The above 3x LB would cost 5700MPM, using the above consumables/buffs thats a loss of 1200MPM, so aslong as you have above 7200 max mana, your IV will cover the loss every 6mins and you will be able to maintain it (unless you have something shit like 200 spirit) indefinatly.
Shaman should add 450MPM. SP I cant honestly say how much they give in numbers, but giving one to the druids and they should be able to sustain 3 LBs + Rejuv with ease (600x3, +233 from Rejuv = 2033HPS over 3 targets).
You cant really do anything more because you will run into GCD issues (7/1.5 = 4.6), and im sure Rejuv would be wasted/inactive a couple of times during the fight if you want to maintain your Lifeblooms as prio (ie, if trinket powered).
2400HPS if your 4th spell was another Lifebloom, perhaps upto 3000HPS over 4 targets if you use trinkets to empower it and maintain it.
But remember getting this is assuming you being non-interupted, silenced, or having all tanks out of range from each other alot (ala 4HM), however some general movement should not effect your performance as each spell is instant.
re

ecimusGarona, I believe people are assuming the ticks of lifebloom, not the end-burst.