mitmproxy does lookup the upstream certificate by default, but at the moment we only transfer common name, subject alternative names and (new) organization name. The good news is that with version 7 it's very easy to add more here, I'd be more than happy to accept contributions. :)
Good news indeed! I tried to contribute 1–2 years ago and quickly realized that such a feature would require a substantial rewrite after a quick code inspection.
