Hi Jude
That is not how I would interpret Public Domain - Google digitized, the following quote is quite clear in not giving you permission to upload content to WikiTree
"The digital images and OCR of this work were produced by Google, Inc. (indicated by a watermark on each page in the PageTurner). Google requests that the images and OCR not be re-hosted, redistributed or used commercially. The images are provided for educational, scholarly, non-commercial purposes.
"
So you shouldn't upload the digitised images from HathiTrust to WikiTree, as although free to use, WikiTree is a commercial website. Some copyright holders have allowed WikiTree to post copyrighted information but you would have to ask permission.
However if you have been given permission by the author and you can access an original copy, you could scan the pages yourself and upload them yourself. I'm definitely not a copyright lawyer but that sounds like it might be OK.
Or as the HathiTrust instructions state - Note: There are no restrictions on use of text transcribed from the images, or paraphrased or translated using the images.