I just found this & am definitely intrigued!
I've had some clients ask about something like this because they're afraid to "mess up" their new website when they start updating on their own. For one or two, I've had to tell them that I could make a copy to have as a backup, which work 'enough' but wouldn't be exact copy as it wouldn't pull over some customizations from tools like SquareKicker in the duplicate, etc. For others, I've copied the pages they were most concerned about and put the duplicates in a folder on their website, turned of search engine indexing for each of the copies, etc. Both of which aren't ideal solutions, but it did help give them peace of mind. 🙃