By chance can the rental agreement email also have a link for individuals to upload their ID directly to Owner Rez?
There isn't a way to upload the ID directly in the signing process -- that is on the TODO list.
If you ask them to email the ID photo to you, you can attach it to the booking on the Files tab of the booking.
I created a Dropbox account with a folder for IDs. I include the link to that dropbox folder in the rental agreement as well as in the confirmation email that I send for each new booking. Dropbox notifies me when a file is uploaded to that folder.
This has been working pretty well. I'd rather they upload their ID to this specific folder instead of emailing it or texting a photo of their ID to me, much easier to keep track of the IDs all in one place.