Well this is my most recent findings on developing windows8 html5 game using scirra.
THe normal XML plugin tries to create new ActivexObject which is not supported by windows8 apphost.
I happened to edit the generated javascript file to use winjs libraries.
Edited Code and commented out for loop where activex object is created
and added the below code
var xml = new Windows.Data.Xml.Dom.XmlDocument();
xml.loadXml(xml_string);
Also comment out the line below
this.xmlDoc.setProperty("SelectionLanguage","XPath");
Please include a update in your next release for fixing this bug for exporting to Windows8.
My changes are not 100% perfect since its changed directly in game script.