A lot of medieval music manuscripts are partially in MBz, but usually only if there’s a recording for a work, and the entries are often pretty sparse and are usually not organized in series.
Wikipedia list of the manuscripts that have entries, I’m thinking that starting with British Library, Add MS 29987 would be easiest, since it has composers credited for most of the works and is well documented.
I assume having a disambiguation for each work would be useful, even if the composer is known.