Nice tip about the X-Entity-Ref-ID usage to prevent GMail from chunking repeated emails together! I will have to note that for my existing web apps.

Any plans to extend this to use GMail's enhancements allowing a confirmation button in the subject line, thus letting the user perform a simple action without opening the actual email?

I see more and more confirmation emails now with the confirm button in the subject line - I believe it requires a few more header specific to GMail to allow this.