This is very cool!  I can imagine using this in order to edit in one tab and preview the web view of this file in another.  

Two small comments: 

(1) If you could host the file at a host that supported HTTPS (e.g., GitHub), this could help users feel better about installing remote code on their systems.  We trust you, and anyone can review the source, but a man in the middle could alter the code along the way.  This goes for other plugins too, e.g., the TEI framework plugin.

(2) The tab for the web view appears to be the URL that it's displaying, but hitting File > Save As results in an empty file, Untitled1.text, being saved.  I see the same result when right-clicking on the tab and selecting Copy Location.  I'm not sure there's a perfect solution here, but it would seem natural that Copy Location would result in the URL being copied to the clipboard, and perhaps Save As could save either the HTML file (as a web browser typically does), or more simply, a plain text file containing the URL itself.

Both these are minor points.  This is useful and a nice demonstration of how extensible oXygen is.  Thanks for sharing it!

Best,

Joe