Though Von Tarnus was known for being a genius artificer, I don't think the Armor of Von Tarnus is so much "an irreplacable relic of lost magic" as it is "really expensive and weirdly specialized in a direction Wizards normally don't go". Heavy armor generally interferes with casting, but it also interferes with mobility... so since wizards generally prefer not to engage in melee, they also don't train in the use of armor, and wouldn't have much use for a set if they did have it. The weirdest thing about Von Tarnus was that he was a Greatsword before he awakened his powers... so when he felt able to do so, he made himself armor that'd let him use his magic alongside his accustomed style, and bent all the skill and resources he had into making that happen. It worked, and the resulting armor was good enough that it still sees use... but it's a special kind of wizard that'd want the thing. Conveniently, Mathilde is the right sort of weird for this, since she's heavily into greatsword melee. Still, there's probably at least one Battle Wizard of that stripe in the Bright Order already, who'll be rather peeved with Mathilde yoinking a famous symbol of their College for her own personal use.
If nothing else, it'd certainly be a far easier project to recreate that armor than it was to make a new Waystone... but I predict that if we try, the results we run into are that no one has because it's really expensive and difficult to make, and there aren't enough Wizards interested in that specific fighting style to make the hefty pricetag of reverse engineering and replicating the thing worth the time of anyone who could manage the feat.