I hope for any answer from the developer of this free viewer. It's worth a try but I have no clue how much time an answer will take...
Anyway I agree that getting Sibelius just for viewing is a bit over the top.
I don't know if you are a proficient html developer - but as I am not I have googled how to basically embed an Java applet into html considering several browsers and platforms as well as deployment of that applet as prerequisite in case we can get our hands on a MusicXML viewer applet.
http://download.oracle.com/javase/1....sing_tags.html