Awesome idea! I’ve been thinking about a similar script but one that works in the other direction. 
I’m also trying to streamline a cumbersome process; creating markdown notes, copying the HTML preview, then pasting into Gmail. I’d like to be able to just add a certain tag, and have a draft email created with the HTML preview. The email address and subject could be pulled from tags or frontmatter, or the filename.
I was going to use Alfred but haven’t had time to work on it. Do you think your script could be modified to create drafts using the HTML preview? Maybe it could monitor a DRAFT folder and upload edits to the draft every so often.