Yes, for sure these things are interesting to preserve! For example, I was just searching HathiTrust and archive.org for Margaret Ogilvy's or Isobel Haldane's letters or diaries...of course, they might still exist somewhere, just not online.

In the specific question, I've pretty much decided that it's worth it to make those notes, anyway. : )

would combining information for the different fics in a single preface work?

I'm doing that for the volume of longer stories, where there are just five or so. But for lots of shorter stories, I think it will be better with a note at the beginning of each one.