Aha, Got it! Okay, for the bug to trigger the dog has to die on the FIRST tick of flame damage as it moves, later ones and it'll die normally. If it dies on the first tick the sound of the dog dying plays and it becomes untargetable but it continues moving for the rest of its turn, and will continue to act on subsequent turns.http://i.imgur.com/Dmfmcjy.jpg
Here in this first screenshot, I was unfortunately too slow to capture the dog dying and continuing to walk around, but you can see it's lying on its sidehttp://i.imgur.com/a3y8mUC.jpg
Second, the dog is now standing, and has just fired an acid blob at my characterhttp://i.imgur.com/wOE5HeH.jpg
and third, it's now back to lying on its side, but in a clearly different position to the first screenshot.http://i.imgur.com/43BLT5P.jpg
Here's a fourth, in which I managed to screenshot the dead dog actually using acid blob.
It only ever used acid blob (whenever it came off cooldown), it never came over and attacked in melee, and once combat was over it went back to being properly dead, although its corpse was still untargetable and it left no remains.
EDIT: Managed to capture the initial zombificationhttp://i.imgur.com/R4E87E0.jpg
dead (note that it's taken a single instance of heat damage)http://i.imgur.com/AEbY1FD.jpg
continuing to move while dead