Christopher Ecclestone has sent me this link which shows that a US university has microfilm copies of all the manuscripts in the Ambrosian library in Milan. Good to know these exist; now what about getting them online where we can see them?
The holdings of the Biblioteca Ambrosiana (named after Ambrose of Milan, of course) are very rich, and include the manuscripts from the abbey of Bobbio. These remained unknown through much of the renaissance, and were only discovered in 1493. The abbey was founded in the early Dark Ages by the Irish monk St. Columbanus, and many of its books were made by reusing old parchment. Consequently the books include many palimpsests of classical texts not known elsewhere.
PS: A sinister note. Apparently the Ambrosian have banned this US library from making microfilms available. All requests must go to the library itself. The library has a site — only in Italian, of course — in which I was unable even to locate the microfilm-ordering service. I think I will write and ask why they do such a thing.