In an upcoming LTT blog post (hopefully, before the end of the calendar year), we will discuss U-M Library's process of enabling page insertions to Google volumes for our HathiTrust Digital Library.
U-M has been doing this work with Google (called SPIR - single-page insertion and replacement) for several years now. HathiTrust has been looking into encouraging more partner institutions to enable SPIR at their institutions, and we've been working with them on this process.
Stay tuned!