According to the XML-RPC Wordpress API Codex, editing a post is the same as a new post, just add the post id and any new values.
So, if a file has been modified, and already has a post id, then I should be able to send wp.editPost instead of wp.newPost.
Time to give it a test:
New function includes the post id and the query action is editPost. As with the other meta data, PostID is culled from the header information if it is available.
Per the recommendation from NoodleSoft I updated the multimarkdown references in the code to use the full path.
It works! This post brought to you automagically with the power of scripting and Hazel baked right in.