I keep wanting your site to work like JSFiddle. That is, I can edit my PlantUML, play with it, and each change gives me a URL to my “file”. I can share it with others, even embed it into my own web page. Source is already zipped into the URL, so it should be easy to integrate that into planttext.com
Regarding “working like JSFiddle”, yes you can do exactly what you are saying. There is an “Edit” link just below the diagram. This link contains the full PlantUML text as a query string item in the link back to this site. In other words, you can give that link so someone so the diagram can be opened in PlantText.com. I hope this helps.