It's hinted at by
the three letters addressed to Cailen that you can pick up in the Return to Ostragar DLC. Apparently there is also some banter with Loghain that discusses it too, according to the wiki, but I've never heard it.
Non-game: Gaider confirmed that it was a
dropped plotline from DAO.
Yeah, the battle at Ostagar is just such a weird/obvious plot device, it's just eye-rollingly bad. I still loved DAO, even with its plot issues - the world that Bioware created is just so rich and alive. And DAO is just a glimpse at Thedas, which is astonishing, given how much content is in that game. Like everything we learn about the Qun in DA2 and Mask of the Assassin (which is an essential bit of DLC, imo) - you barely learn anything about the Qun in DAO.
DA's world is one of the most interesting fantasy universes created in the past decade, imo. Given that it is the background to a series of games (primarily) is just amazing. Kudos to Bioware for creating it and kudos to EA for letting them do it.