Which medieval writers are you thinking of? Heloise is one of the most elegant stylists of any age, imo, and Abelard's not bad, either.

There are grammar commentaries as early as Caesar (De Analogia) and certainly by the 2nd century (Noctes Atticae). I have fuzzy memories of reading authors from the Republic or Empire writing about the ablative absolute, but those notes are in storage somewhere. :/