Add-ins can work on iPad, however adding them to the device is limited to side-loading via iTunes or those listed on the Office Store. There needs to be a more direct way to add add-ins to iPad Excel as these two options have limitations.

Sideloading is difficult as it requires iTunes and it seems that everytime that the add-in is loaded, it pulls the web-app from the server so offline use is not possible.

The Office store is not good for developers who are not making add-ins for the public nor their personal company, but clients. It is not possible to load a private add-in onto the Excel iPad verison at this time.

Adding a simple URL entry field that lets the user specify where the manifest file is located would go a long way. That add-in would then be cached to the device.