This is, funnily enough, something I've been periodically searching for for nearly a decade, and even renewed my search yesterday. For those wondering, my use case is a model Parliament (context: https://news.ycombinator.com/item?id=43474850) and deciding on using git to store the laws, with the commits representing the Acts of Parliament themselves. The problem is that we need to guarantee that every commit has the correct metadata: we do not care at all about the conservation of commit hashes. And if we've accidentally missed a law and need to interactively-rebase it into the git history, it should NOT reset all the subsequent commit dates to the current timestamp. Basically, we're wanting to use git as a historical archive. I'm delighted to have found this.
Fwiw, in plain git, there's 2 dates associated with commits. The author date tells when the content was authored, and doesn't change with rebase. Only the commit date changes. So the correct metadata is already there, and even more precise. It can tell you both when it was authored, and when it's history was last changed.