I must write some plain text that's of indefinite length, and could eventually achieve 10,000+ words. At any time others can easily see the progress from the text, page by page. The written text do not need to be editable once it's been written.

Would I be best storing each page (~100-200 words) in MySQL rows, or each page like a plain text file?

Generally when storing 'files' the party lines are to place the file on disk and store a hyperlink. As you are speaking about 100-200 words in every file I'd be inclined to simply insert them in a text area and store each 'page' like a row. This could provide you with the capability to view updates faster (faster than storing all 10K words at the same time). You'd need to store information on page order also simply to keep things showing up properly.