In ViewerConfig, add these viewer properties in HeadJS:
viewer.MobileArticleViewerDefault = true; // this would already be set to true for existing mobile article clients
viewer.MobileArticleViewerCustomUrl=[URL-HERE] //this is the new property
EXAMPLES:
new article viewer:
viewer.MobileArticleViewerCustomUrl="/global/mobilesite/index.html"
wordpress site (example)
viewer.MobileArticleViewerCustomUrl="http://mygreatWPsite.com"