Changing the ActionMailer Preview header to include more metadata

I’ve been struggling with adding metadata to the ActionMailer Preview-header for a while.
What I would ideally like to do is to override the following file somewhere, adding some custom metadata to the header like in the picture attached below:

Alternatively override the mailers controller to render a different template on preview:

Is it possible to easily override either of those files without having to change the base gems or similar? Would an extra initializer be needed and if so what should be included?

Hope someone here has some insights!