Simplest solution will be (using jQuery):
$('a[target=_blank]').on('click',function(e){
e.stopPropagation();
e.preventDefault();
window.external.notify($(this).prop('href'));
});
In Windows 8 you must add your page (host) to AllowedScriptNotifyUris
In Windows 8.1 you will probably need to have some other workaround to use window.external.notify
as allowed hosts are embedded in app manifest and must be loaded by https