There is only one thing I can think of, and it doesn't manage any inline markups like <strong> or <em>, and doesn't manage headings.
You can take generic blocks of text with a line space between, like you're seeing in this post, and with your scripting identify the text blocks as paragraphs. Then when output, you have <p>content.</p>
But there **has** to be some form of "marker" or something in the text to designate headings and other markup. BBcode is a good example.
Perhaps you can do something that stores the markers/html and when viewed or exported for whatever reason, you strip them out. So the HTML is there, but the client will never see it. Kinda sneaky, but meets the requirements and the site still works.
Of course you'd have to do the same thing in an edit, and lose all the markers in the process.