Does a <![CDATA[ … ]]> section fit your definition of a no-markdown island?
You can put almost anything you want in one, except maybe ]]> There seems to be a few fine points to work out around them, but they are already in the spec.
I was testing in the block context and found a spec issue, see: Drawing a distinction between HTML block elements and non-element tags. We may need to pay similar attention to the difference between Raw HTML and non-element tags like CDATA.