For all those who weren't there: The Poor Unfortunately Souls run was slick and smooth. The achievement timers allowed for an hour total, and we completed it in thirty minutes. My thanks again (in print) to Ghoselle, Andius, Snique, and Jenilea.
I'd like to thank Winston for organizing this, and for doing all the research in to everything about this achievement. He did the preparation so that if it'd been much harder we'd still have had a good chance of success.
And my offer still stands, that if there are people who want this pet, I'm willing to tank this again.
What we did:
gathered at Maw of Souls
cleared the trash in mythic maw of souls up to the first boss
exited the instance
group queued for heroic black rook hold
in the first room, click on the blue candle/brazier (This starts the timer)
completed heroic BRH
left instance group, which put us back at the Maw of Souls entrance
cleared mythic maw of souls
checked mailbox and got pet
This gave us lots of time for every step. We could have skipped the trash clear for maw, but there is no reason to do so. Each boss you kill in Maw resets the timer. The timer is relatively generous at current gear levels. Folks world quest geared should have no problems with it.