It appears that multimarkdown has a feature which converts two consecutive hyphens -- into an emdash —.
For example, in the Byword editor, when you type those two hyphens it automatically converts to an emdash.
This is fine, and very useful for those writing longform text. However, it gets in the way when trying to document a SQL statement which includes a comment.
Is there a way to escape this behavior so that it shows up correctly within a fenced code block?
Your question is actually flawed in its premise --- there are no "fenced code blocks" in MultiMarkdown. So this is not a MultiMarkdown question, it's actually a Byword question, if that's where you're using it, or a question for whatever other markup language you are using.
MultiMarkdown itself does not convert dashes, or other characters like that, in code blocks or code spans.
Dashes aren't converted here --- see?
And they aren't converted here either --- see?
(And yes, I realize that this site isn't actually using MMD, but the output would be the same.)