Seeing that Zurb released a ruby gem version of foundation for email I’m wondering how to approach integrating this with middleman. Any ideas?
While not a direct answer to your question…
I used Middleman a while back to generate custom transaction emails for a Magento project. It’s based on Bootstrap (not Foundation) but it should point you in the right direction: https://github.com/hello-jason/magento-transactional-email-generator
You should mostly concern yourself with the gulpfile and the premailer package.