I've just asking myself...
The Harenarius is just like lead-worker and it's one exclusive House Virtue, or if it's one possibly Virtue outside the House tremere. I am making one Fambeau Duelist-for-hire and it's very attractive to that kin of character. If not, I am thinking on Warrior to represent his martial needs (and Special Circunstances and Necesary Condition - to sustain one sword anyway).
I'm not familiar with the Harenarios Virtue - can you note which book it comes from?
Leadworker certainly isn't exclusive to House Tremere. It's more common within the house, but Tremere isn't a mystery cult or built around a specific bloodline, so they don't really have any funky magical secrets of that nature.
They make up for it by having entirely different kinds of secrets. Less like mystical cults and more like the NSA.
That virtue, allowing someone to master two dueling styles instinctively, is absolutely not Tremere-exclusive. It's just associated with Tremere because Tremere are the House that focuses more on certamen.
Truely, the book is the True Lineages, on the tremere Chapter, but while the Lead-worker is not declared just there, thera other sources where that Virtue is exclusive to the Tremere. It isn't one Mystery, but one legacy magic from their Necromantic foundations. This is not one really widespread Virtue, neither in House Tremere.
"I hear you've been teaching your apprentice how to make those strange Arcane Connection figurine thingies. That's great! I'll just take him for the good of House Bonisagus now and make him show my other apprentice how to do it! Oh, and I'm sure that he can send a tractatus on the technique to Durenmar, of course."
"I'm sick of you and the Exarchs telling me what to do and pushing me around, parens! You don't understand what I'm going through just because I like to wear black and talk to ghosts! I'm going Orbus! And I'll teach EVERYONE who wants it to make leadworker figurines!"
That said, it's probable that it simply hasn't spread around.
But i can imagine than one Tremere directed campaign against that examples are equally posssible... and Interesting.
I am making this character like one of one series of generalist magi (one in fat is one Tremere maga), and I am thinking about one Didne user too. If tat would be one player character, then i am less conservative about that, that Virtues makes stories easily.
Well, if a non-Tremere character having Leadworker fits the needs of your story, go for it (a Titanos would just love to have that skill, for example).
The only Hermetic Virtue I can think of that is actually hard-restricted is Mutantum, and that's because Mutantus was a mutant and Mutantum Magic is limited to the vulgar bloodline of Mercere instead of the Hermetic lineage.