So I've managed to find the magical item ID for 2 for 1 Zinger burgers. With a huge thanks to cooni as the original finder of this method of hacking the menu.
2 for 1 Zinger Burgers Link
Troubleshooting
If the links open in your browser (not the app) and you just get a 404 Not Found error,
iOS:
Long press the link and tap 'Open in "KFC"'. If this option isn't present, see if there's an app update available in the App Store.
Android:
click the Red KFC icon in the url box to get redirected to the KFC app
For color os / oppo users… heres how to adjust your app settings in order to use the links to take you directly to the app.
While you're at it, find the one for $2 large chips