A very interesting idea, no doubt. We were discussing this since the early days of the Vienna Symphonic Library, almost a decade ago. We were even asked at some point to sample the few still existant original instruments of W.A. Mozart at some point (yes, the ones he played himself), as some kind of cultural heritage project ... This exciting idea got buried due to some ugly "political" conflicts of competence between the two Mozart-societies in Salzburg and Vienna, as far as I remember ... but I degress.
The main problem is that there are so many "schools" of how these old instruments were actually played. Another issue is that good (and authentic) instruments are rare. Not an easy task, in any case.
Never say never, but I wouldn't hold my breath for a VSL "Medieval" collection ....