Maybe I’ll ask an obvious question, but what prevents someone from opening the markdown file with some other editor and making changes? Or is the point to somehow also hide the markdown files so they’re not accessible outside the “viewer”? This to me sounds more like an export option to HTML as described here: Local Publish/Export Package: Convert contents of a vault into a single, browsable HTML5 (or other) file for secure sharing with non-tech-savvy clients
Edit: Just realized that both posts are from @joeshirley, which explains why they sounded similar! AFAIK the HTML export is on the pipeline (although not as a single file), while the viewer from my point of view might be harder to implement.