@colinwalker Hah. Nice. Well I ended up at both the concept and my RegEx independently. Which suggests we're both working along the right lines. Actually... Hmm...
@colinwalker No, "Hmm"s are usually good here. Means you've given me an idea. Which has worked! :)
@colinwalker You just helped me fix the RegEx. I was making it more complicated than it needed to be. The plan for that plugin, by the way, is to add a "Quick Post" Dashboard widget - similar to wpquickimage.com - with a word count, to help posting from inside WP.
@colinwalker To be honest, I was mostly editing inside WP to make links work! Now that works I can probably happily use the app.