Markup: Conditional rendering for some filetypes (literate Haskell)

Following on from the discussion at #266 and a Reddit thread, I'd like to try getting literate Haskell rendering back into GitHub in an opt-in fashion. The goal is to only render .lhs files as markdown (via the literati library) _if_ they contain a specific HTML comment (<!-- literate-haskell: markdown -->). I figured on doing something like this:

markup(:literati, /lhs/) do |content|
  if content =~ /^<!--\s*literate-haskell:\s*markdown\s*-->/
    Literati.render(content)
  else
    content
  end
end

However, how will this interact with GitHub's content rendering? The goal is to have files completely unrendered if they do not contain the formatted comment.