I have just become aware via this site that quite a number of the letters in the Collectio Avellana, a mass of 243 papal and imperial letters of the 5-6th centuries, may exist in English. There is a three volume collection by P. R. Coleman-Norton, Roman State and Christian Church, 3 volumes (London: SPCK, 1966), which lists a good many of the first 50 as translated (in the rightmost column of the table).
Of course the question is whether letter 100, on the Lupercalia, is one of them! If it is, I may need to get some of these Latin translation people at work on something else. Maybe some letters of St. Jerome or something.
I was intending to go to Oxford tomorrow, but perhaps a trip to Cambridge and some library time might be a better option.