This could be done by editing language settings by going to admin / settings / languages, and doing a search for $site_name. There will be several instances pull up, and several will not be email related. When you find the email, or emails that you want to change; replace the ($site_name) with what you want to display. You will notice that all emails have a text, and an html version. You would change this in both versions. I have never tried this myself. If it did, I would change only one email, and test it to make sure the change is what I was looking for before changing others.