Mirrored repositories won't automatically mirror for you, but it will display as mirrors do on GitHub:
- Mirror link displayed under repo name, "mirrored from "
- Pull requests and issues disabled/hidden
For an example, see https://github.com/chromium/crashpad
The easiest path on the lexicon side would be to add a mirror string (URL format) property to the repo record, and enable mirroring if the property is present