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.
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();
Also comment out the line below
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.