I think between the examples you already know of and the ones Andrew mentions, you already have all the major institutions on Wikisource in English. The Archives of American Art has had language on its Wikipedia WikiProject asking for transcriptions on Wikisource, but only a few pages have been done and not in an organized way.
Though NARA is a large institution, it may be interesting to note that all of the work it did on Wikisource was the result of a little time put in by an intern (me), and no full-time staff aside from the work that had already gone into digitizing and describing those records. And the Wikisource community, of course. One of the main innovations we developed was the "Transcribe" button which is on all NARA image uploads on Commons and preloads the Wikisource transcription page (
see it in the wild).