Today I was at work reading a little of Societates and I start to think: Someone has play with the idea of other Houses training hedgies and how much they will do it?
Ex Miscellanea works on the idea that your tradition is maintained in some form (Major non-hermetic virtue), but also the Minor Hermetic Virtue implies that your tradition bleeds inside the Hermetic Theory some benefit (and problem in the form of the Major Flaw).
But, someone has played with this idea on other houses? Paying with your normal virtues instead of using the free ones of Ex Miscellanea. A Tremere training a Elementalist capable of extracting Elemental Vis? A Bonisagus training a Learned Magician on how Hermetic Magic is inherently better? Etc, etc.
Early on in the Order (probably only during the life of that founder but maybe a little after), even the True Lineages did this. The only canon confirmation I can remember is the Aita priests brought into the Order by Tremere but that example is enough to prove it happened but even in the 1220 era I can image it very occasionally happens that a Societes or Mystery Cult House sponsors and inducts a hedge wizard directly into their house instead of them joining House ex Miscellanea. So I think it is plausible. Another more recent canon example of something like this happening was the lineage of Goetic summoners brought into house Tytalus to replenish their ranks after The Betrayal (known as The Corruption to the other houses of the Order) though I think they joined out of ex Misc rather than from outside the Order and by 1220 are generally created as normal-ish Tytalus.
I have sort of done this though, as the tradition was one of the full Hedge Traditions within House ex Misc, I did not take a minor Hermetic virtue or flaw linked to the tradition. The character was a Criamon with the Control Fertility ability of the Scinnfolk. Her backstory was that she was the child/apprentice of a member of a branch of that tradition that was not in the Order and a Criamon came along and took her as an apprentice. 3/10, would not do again with Control Fertility. I liked the character but Control Fertility didn’t really have much synergy with the hermetic side of their magic and, as she was a Creo specialist, there was heavy overlap there and only a few uses of that ability were things she couldn’t do using hermetic magic so it felt somewhat like a wasted major virtue and the xp needed to put into it. If the campaign had gotten further it may have been useful as an insight source for Fertility Magic or something but the saga didn’t get that far.
I’ve also made a Bonisagus who started their apprenticeship trained by a member of the Lineage of Pralix and had the full suite of V/Fs for that lineage bought with the normal limit of 10 V/Fs. I felt much better about this character after creation but never played them though I knew I would not be disappointed with having Comprehend Magic since it’s a great Sup Ability and I had played an actual member of that tradition before.
I’ve thought of a few others along these lines but generally ended up just making them an ex Misc instead. Those traditions were Columbae (again a bonisnatched apprentice though I also thought of a Columbae who switched from ex Misc to Tytalus to join the Titanoi), Witch of Thessaly (again joining the Titanoi) and a Haruspex (Criamon).
Just like when selecting a lineage in ex Misc or even just taking a supernatural ability for their magus one needs to think carefully whether there are either synergies or taking that ability gives them something cool they otherwise couldn’t do or couldn’t easily do early in the game. For example Shapechanging is a great ability but it’s lackluster on a MuCo(An) specialist and is only really cool for a magus if you aren’t going to be focusing on the arts that would allow you to shapechange or, as with Magic Sensitivity or Second Sight, the cost is low and there is a significant benefit over Hermetic means of doing that thing
There really isn't anything preventing a non Ex Miscellanea from recruiting Hedge Wizards into his House. It is somewhat unusual by 1220, granted, but as pointed out already, this was a common practice in the early days of the Order. Recruits had to come from somewhere! More then likely this would only be considered if the Hedge Wizard has a certain resonance with the House philosophy or magic.
Tytalus have notably recruited a tradition of Theurgists from Ex Miscellanea (also noted above), but they are also a house that recognizes "dual membership" of sorts. I'm sure they'd be all over a Hedge Wizard that could sufficiently impress them.
House Flambeau has at least three groups that can trace their lineage to non-Roman traditions.
-Hibernian covenant of Lámbaird hails from a local Druidic tradition, which specialized in dispelling magic and consequently the Flambeau from this covenant practice a unique school of combat, which is all about suppressing Magic Resistance (The School of Raghallach).
-Montverte in Normandy is home to a lineage of Flambeau descended from Harivald, a viking sorcerer skilled in elemental magics. Assuming their founder was an Elementalist of some variety, it is unlikely that much of their original magic was preserved, but not impossible.
-And then there is a small group of Zoroastrian Holy Magi. Holy Fire + Flambeau = Profit!
So, while such occurrences can be rare in this day and age, it is far from impossible. For example; the Crintera Schism scenario could potentially result in the Harmonist Bjornaer fully absorbing the Werewolf Clans of Pomerania into their ranks. Another prospect would be the Learned Magicians. The nature of this tradition makes them well suited for both Bonisagi and Jerbiton magi, should they take an interest.
If you want to design a Hedge Wizard who joined Houses other then Ex Miscellanea, you can always just use the rules for Magi Orbi; design them following the rules for one House (Ex Miscellanea template in this case) and then just stuff them into whatever House makes most sense. Or just design them per the rules of their Hedge tradition if they never adopted any Hermetic Magic at all. For Flambeau you can always use the variant House Virtue rule to represent a specialized lineage (as in the case of the Flambeau of Lámbaird).