git format-patch has --notes "to write supporting explanation for the commit that does not belong to the commit log message". That explanation goes after the three-dash line (---) but before the diff stats. It would be nice if submitgit was able to mimic this feature (maybe optionally, by ticking a checkbox in the web interface).
As a side note, unfortunately GitHub makes it hard to double-check from its web interface that a commit has notes attached as they disabled displaying of notes in August 2014.