You can set multiple email addresses on your GitHub account, and can route notifications on a per-org basis. However, when you create a commit through github.com (either through the web editor, or merge commits generated by the green merge button), the commit is always authored by the user's default email address (unless of course they have email privacy enabled).

I would like to be able to set the default email so that any changes made in a github.com/google repo always uses my @google.com email. (This is particularly important for us because we want employees to always use their work email address for Google repos)