Interesting thing that happened to me a few weeks ago:
Was running CT as a BLM, during 3rd phase of Bone Dragon the MT went down and he ran over to me as #2 target. I got the marker for chaos breath so I popped Manawall on a delay hoping that it was physical damage. I didn't see my hp drop from the attack (but didn't think to check the logs to see what happened, didn't know there was anything special to the attack other than giant red mark of doom). Another tank then provoked off of me and we killed him a bit later. Would be fun to set up a situation to test this again and see what happens. Also interesting that a BLM may be able to block the damage but HG or Holmgang can't.