I’m not a medievalist myself but I have done much research and font work upon medieavist and linguistics-related characters. I don’t see why it shouldn’t be a worthwhile project to provide these scholar groups with well-equipped fonts in the style similar to Times or Caslon. I’m pretty sure that some potential users would welcome such a choice – for which John’s Brill is a very good option already.
I circumvent this cumbersome glyph-substitution business, still, with regard to small caps. I do the small caps as a seperate font (yes the old-school way) because I like to keep things simple and straightforward. In the sc font, the position of lowercase ß gets a small cap ẞ glyph. That’s logical and no one complains.