http://dishonored.wikia.com/wiki/The_Brigmore_Job

Foreshadowing found in base game for "The Brigmore Witches?"

Definitely a reference to Daud's story in both DLC packages, but not an introduction to "Brigmore Witches" specifically. We were told that one will have Daud as the protagonist too, right?

Yep.

So I personally think Birgmore is going to be after the base game.

I think it will be before the end of the base game (I see that Arkane are somewhat avoiding defining any specific outcome as canon so I'm pretty sure they will try to wrap it up before Corvo gets there) and Daud will write that note as a result of what happens in the DLC. Not sure tho.