Not sure if this would work, but it offers to let you "upload" from an existing Google Drive folder. I wonder what would happen if you added the >50MB file to google drive, and then selected it from its Drive location in the books uploader UI.
The largest files I have to test area 40MB PDF and 35MB Epub, which worked. I tried a 100MB iso renamed to pdf, but it seemed to detect the right mime type and excluded it