Sadly not - the mechanism employed in the add-in has extremely limited scope: desktop Outlook is the only client I'm aware of which pays any attention to the flags being used here. IRM is pretty much the way to go here, which covers all clients.
Having said that, a possibility is to write a browser add-in which recognises OWA pages and adjusts the HTML being rendered to remove/disable buttons - but that would be a very fragile method, and fairly easy to defeat. Not something I'd fancy writing!