> I'm unsure whether SQLite's extension interface is flexible enough to support this.

I think it is, if I've understood your requirements correctly. e.g. from the datasette-faiss docs:

  with related as (
    select value from json_each(
      faiss_search(
        'intranet',
        'embeddings',
        (select embedding from embeddings where id = :id),
        5
      )
    )
  )
  select id, title from articles, related
  where id = value