When I talk about PoMending hops from tank to life tap....
1) Lifetap; Warlock Spell; Drains x amount of health from Warlock converting it to mana.
Tank would take damage, PoM procs, hops to a lock. Lock Lifetaps, PoM procs, hops to either the tank or another lock.

I agree that mage would add some more utility. As for spec'n the locks demonology... I did this at 54 way back when. I was killing less mobs per minute, with higher downtime. Pure affliction is the best drain tanking PvE setup, and pre-TBC it will enable to you to tank 5-6 mobs at any given point, while maintaining 70%+ hp/mana utilizing instant Howl of Terror, Voidwalker holding agro on 1-2 for 5-6 seconds, Death Coil, single target fear, drain life, and the 4 dots you have available (most times I would skip siphon life as it is mana taxing). Also note, I twinked level 39, had somewhere around +250 shadow damage, and used the SAME gear until I hit outlands. Baller, I know :P

For destruction to be effective, you need +202 hit rating, 20%+ crit, and ~900 +dmg.
The spec is something similiar to this HERE

With three locks, you could change the levels of Improved Healthstone to be 0,1, and 2 of 2, giving you 3 healthstones per toon.
Generally you sacrifice your succubus using demonic sacrifice, gaining 15% shadow damage. Then you spam shadowbolts.

Raid proven the highest DPS spec.