I found a very late (1918) letter that was sent along with a historical manuscript, entitled Toledot Yisrael, describing the circumstances under which it was found (under a rock during the shelling of Jerusalem!!) and donated to the library. Unfortunately, there was nothing earlier along those lines, and certainly nothing like this from the pre-Neubauer period, before the 1886 catalogue that contains the entry for my manuscript was published.
One interesting thing is that in the early 20th century, the library was willing to send manuscripts out on loans to other libraries (that, of course, all vouched that they had suitably safe and fireproof places to store them).
There were a number of other somewhat interesting letters in this folder. I'll hold off posting them for now for a really mundane technical reason, though, namely I'm on a ridiculously slow internet connection.