Having had this problem, I can confirm that the following worked:
> Reload to just before destruction of Haven
> Play through to reaching Skyhold (I selected the "we must stop Corypheus" option)
> On reaching Skyhold, I spoke to whom I liked, including Dorian (who had no additional dialogue at this stage). I then unlocked and fast travelled to Crestwood, walked around for a bit (reached the village) and returned to Skyhold.
> I spoke to Dorian and the letter scene triggered. On finishing the conversation, Giselle was downstairs in the hall with the quest.
Unfortunately, it seems this quest needs to trigger early on after reaching Skyhold; as to whether or not it relies on other dialogue options is unclear, although I have to say that judging Alexius seems to have had no bearing on it (as I haven't judged him at the reload, and whatever I did in my immediately-pre-Alexius prior save couldn't trigger the scene). As a side effect, it does seem that leaving it too long will prevent this quest from ever firing properly.