On Github RyanParsley / html-email-presentation
Thunderbird has a super helpful "insert HTML" feature that does what you want it to.
A static site generator built with Sinatra.
I took the liberty of setting up middleman to generate html emails formatted with Zurb email templates https://github.com/RyanParsley/mailman.A gem that will parse CSS in the head of your document and inject the styles appropriately.
Most of your middleman bits are transferable to Rails
GitHub deploys can trigger a build
On GitHub, set up a WebHook URL pointing to an app on Heroku. That app can run a build and deploy to a second app on Heroku. Content can be edited in GitHub's web editor or with the super slick Prose.io app so content editor's don't need to use git.