Rather insightful observation, OP. Even sleep deprived, it none the less had me pondering a great many things concerning Mage Hawke; which, FYI, is something I'm looking forward to. One of few things which irked me about the Origin's approach in the first Game was the fact that I could not be a Mage with-in the Origin that had the best back-story and was the most *canon* of all the others, the Cousland Origin. You had to choose between being either a Warrior or Rogue if you wanted to follow that particular plot-line. FUBAR I say.
In Redcliff, the Warden would come to learn that Lady Isolde hired an Apostate, or as it's refered to in Awakenings an "Apple-state" for whatever reason (giggle), to train her son Connor in secret for fear he would be taken by the Circle and the Chantry, there by loosing his family and being severed from his inheritence and titles. What struck me after your post, also being something I had thought of once before during a previous run through of DAO, was that though Isolde kind of screws the pooch with regards to who she picks and what it all leads to, she could not have been the first Noble, decent Human Being or otherwise, to have taken this Route to preserve their Children/Legacy. Simply because Bryce Cousland was the epitome of what a *good* Noble should be doesn't mean he and Elenor would have been adverse to choosing a course of action similar to Isolde's in regards to their youngest child. It would have seemed not only believable to me, but would have fit with-in the established laws of Dragon Age's game-world had the Cousland's chosen to have their Mage-child trained in secret; perhaps by a Hedge-Mage/Witch Bryce encountered and faught alongside during the War with Orlais; a person who had proven to be of great moral character despite never having adhered to the laws of the Chantry or Circle.
Sadly, the Origins decided to be somewhat...restricting. I daresay more so than what we have in DA2. Yes, you can only be a Human PC in DA2, but at least it's a core Origin that isn't restricted by class. I can be a Human, Pure of Heart (as much as the story will permit) and be a Mage outside the Circle. To accomodate this I think there is a good possility that both Hawke and Bethany might have been instructed by a Hedge Mage/Witch. Who this man/woman is, if that is indeed how our sibling pair were trained, is a mystery, as is whether or not BioWare picked a backstory similar to this for those who choose Mage as Hawke's class. At this juncture, it's really all up in the air. Regardless, I'm just looking forward to seeing how it all plays out; however, I am hoping to discover that Hawke is not a Mage bound by the Circle or Chantry. Though I do not dislike either institution (one attempts to keep Mages from going all Abomination-y while the other feeds and cares for the sick and down-trodden) it would be nice for Hawke to stand-apart, as well as offer a deeper narrative for the Mage hero. Just my two cents.