In Pandoc, line blocks are used for preformatted text (<pre>).
Discussion relating to line blocks in CommonMark.
In Markua, A> is used to represent an aside, with other letters combined with > representing different types of asides (e.g B> for blurb, W> for warning). Perhaps something similar could be considered for CommonMark. Although using a more generic approach might be preferable, such as block directives.