Have you considered having a detailed version history for each book (etext)? The process of submitting fixes to typos etc in books involves sending an email (https://www.gutenberg.org/help/errata.html) and although the last time I did this (2011) the fixes did get applied reasonably quickly (couple of days), it all felt a bit opaque. The version history could also include the project (usually PGDP correct?) the etext originated from; that way one would be able to compare against the actual page scans.
I have very mixed feelings about Standard Ebooks and would much prefer being able to use Project Gutenberg directly, but one good thing Standard Ebooks does is that every book has an associated git repository (on GitHub), so it's (in principle) possible to see a history of fixes to the text over time.
I believe our new-ish CEO Eric Hellman actually did some work on something very similar
That's an interesting idea. not a small feat to accomplish though ...
We're using git repos internally to keep history for each book. They existed on github for a while, but our implementation was awkward, and too big of project for the volunteer dev team. But it's likely that we'll evolve towards that.