I think I agree with all of @jonschlinkert’s comments here.
But since it will presumably be common to have “hybrid” documents with some metadata at the top, followed by CommonMark text, I wonder if it would make sense to have the spec define a recognizer for front-matter metadata, so that all CommonMark parsers would know to skip this and go right to to the text.
For example: if the document starts with a line containing ---, skip to the next line containing just --- or ..., and start parsing CommonMark after that.
This would fall far short of specifying a metadata format. Between the opening and closing metadata signs, you could have anything you like – so, you could use YAML, or JSON, or XML, or lua tables, or a custom key-value store. Parsing this would be application-specific, but conforming CommonMark parsers would know to skip it.
The advantage is that, with this feature, you could run your hybrid metadata/CommonMark file through any CommonMark parser and get good results, not the garbage that would result if the metadata were parsed as CommonMark.