This idea amuses me, along with an attempt to alter the mindset of the house. It might come from the covenant of Thousand Caves (4th Ed, The Dragon and the Bear) that has many mundanes able to express their Heartbeasts, somewhere in the Urals.
This would, at the very least require original research and a breakthrough. Would it need to be a Major Hermetic Breakthrough or something lesser? A high value in Bjornaer Lore might be a necessary part of the research, as it seems to be a flaw in the proper integration of Bjorna's magic into Hermetic Theory. How might this be achieved? Sources of breakthrough points?
Given the "slightly" recidivist nature of the house as a whole, there's likely to be some backlash. Dark Secret flaw seems de rigeur, ideas/suggestions greatly appreciated.
Mechanically, it comes down to taking the Initiation Script for the Ritual of Twelve Years (HoH:Mystery Cults, p.26) and modifying it, per The Mysteries, p.17.
Which, distilled, would come down to the Mystagogue, when initiating someone new to the Heartbeast, having to make an Ease Factor 12 (Int + House Bjornaer Lore) roll to substitute a different Ordeal, or an Ease Factor 15 (Int + House Bjornaer Lore) roll to omit the need for the Ordeal entirely. After which, if successful, you have a new Initiation Script that can be shared.
Socially, well, if House Bjornaer were open to members who didn't give up their ability to have a familiar, given how relatively easy it is as a mechanical matter, I would expect it would have already happened. So, yeah, Dark Secret and the like.
That Bjornaer magi can't take a familiar is because that is an Ordeal that is part of the Ritual of Twelve Years - the initiation into the Heartbeast Mystery.
So it should be practically impossible for an existing Bjornaer magus to ever get around that so they can bind a familiar, but by using a different initiation script it could be possible to let future Bjornaer magi bind familiars.
But since this would mean changing long-standing traditions of the House, it would mainly be a political process rather than a question of magic research.
I can see the adjustments to initiation scripts working only if Bjorna's magic was properly integrated into Hermetic Theory. I have a memory (maybe a false memory) of her having a problem with Hermetic Theory where it applied to familiars and the Heartbeast, so, making a virtue (flaw) of necessity, lacking the ability to bind a familiar became part of the Ritual of Twelve Years. If this is not the case then only the invention of a new script is required. In itself, not just a hand wave.
If my dodgy memory is correct, research will still be required. As things are, no familiars for those already a part of the house, but future magi could bind familiars with a different RoTY. A Bjornaer with a magical human, or even a Jinn as a familiar? Might disturb things at a Grand Tribunal if the "animalistic Bjornaer" arrived with a human familiar, especially if it had originally been a mundane human whose Heartbeast had been opened and then bound as a familiar.
While it took some time for Birna to learn Hermetic Magic, she had no special difficulty with any particular part of it.
Merinita was a mentor to her, and helped invent the Ritual of Twelve Years. Merinita was the one who brought the knowledge of how to bind a familiar to the Order, so it was not for lack of knowledge or skill.
No, Birna chose that as an ordeal simply because she wasn't interested in binding a familiar anyway, so she thought it a small price to pay.
If you have access to Houses of Hermes: Mystery Cults, on pages 24 and 27 it mentions the fact that Birna didn't have any interest in binding a familiar, and thus didn't mind the sacrifice when she designed the Script, and one of the story seeds mentions a Bjornaer who has adapted the ritual for mundanes, and thereby changed the Ordeal, since they can't bind a familiar in the first place.
Those were the only examples I knew about. I find it hard to believe that only three of the founders picked up on this major advance in magic, but I agree with "we don't know for sure".
In particular, it seems really unlikely that Bonisagus didn't try out every kind of magic he could. I would guess Flambeau and Jerbiton as well. Probably Pralix would, being a practical sort, but the other founders I can see arguments against. (I can also imagine that Pralix treated her familiar as a military secret, so nothing was ever recorded about them.)
Jerbiton and Mercere both had cat familiars - Jerbiton's was the founder of the Black Cat lineage mentioned in Realms of Power Magic (pg 70), and Mercere's of the White Lineage.
As Salutor said, Jerbiton and Mercere had Hermetic Cats. It is also hinted at that the Tortoiseshell lineage had links to Diedne and so there is the possibility that she had one as a familiar (though that is just guesswork).
Tremere had a wolf, or at least I remember him having one in earlier editions.
Thank you all for lifting the scales from my eyes in this regard. Clearly my memory was/is at fault in this. I can now wander off and plot a story of slightly aberrant Bjornaer in the Urals.
Is there any reason that a mundane human cannot be bound as a familiar? The church might object, but that's just what they do anyway. Likely to be plenty of pagan types in them there hills. More concerned about whether anyone in the house/order would be annoyed enough to object strenuously.
Unless something unexpected changes in DE, a familiar must be a beast unless the Magi has V/F which allow other types to be bond. However it is perfectly alright for the familiar to have a power or enchantment that allows them to change into a 'human'.
In general, hermetic magi can only bind magical animals as familiars.
For anything else (like faeries or spirits or plants), you would need some specific virtue or special knowledge.
One of the House Rules I have for my saga if this situation is approached is as follows:
Birna (aka Bjornaer) started out with an infernally aligned Tradition of shapeshifters out of Pomerania.
Birna received epiphany from her Ancestor Spirit that the animal shapes were cruelly stolen form other Ancestor Spirits.
Birna learnt to embrace and welcome her HeartBeast, the other half of her soul.
Bjornaer's followers also embrace and welcome their Heartbeast as the other half of their soul.
Merinita brought secret of how to bind a Familiar to the Order of Hermes.
You mutually respect and welcome an animal who becomes your other half after binding.
The Hermetic Limit of the Soul prevents you from having more than 2 halves of a Soul.
A Bjornaer can no more have a Familiar than an ordinary hermetic can have more than one Familiar.
There is probably also some magical metaphysics about the balance between a human soul and a beastly nature.
To get a Bjornaer Familiar you probably need to explain away at least some of the above.
Magical Beasts sometimes accept Familiar bonds for their own benefit.
If there is no friendly mage, maybe the mightiest magical creatures might attempt to bind a human as a Familiar? After all, sometimes a dragon merely kidnaps a princess.
Perhaps, just perhaps an old Bjornaer maga might want a Familiar for when they go Great Beast?
Does the above imply that any person without either a Heartbeast (the clearly superior other half) or a familiar is somehow lacking a complete soul? How do mundanes and church folk cope with this? There must be many unrequited/unfulfilled souls kicking about.
Took a while for the implications to percolate through...
I think it's less "Lacking a complete soul" and more "The complete soul is made tangible in one form or another." Grugachanan and Nightwalkers have similar things going on. The frequency by which this becomes a thing implies that there's SOMETHING to it, and if anyone does a full survey and notices that it could certainly cause some upset. I imagine some theologist would write a lengthy thing about how Adam was given dominion over animals in the Garden of Eden and make it all okay.