How to add text to the standard iPad wallpaper

We have thousands of iPads in our business, all controlled by MobileIron. We would like to put a message on all the iPads pertains to password policy.

The way we propose is to take a copy of the standard wallpaper, add the message to that using a Paint like program, then use that as the wallpaper for the lock screen.

However, we can't find a way to add a wallpaper to those which are stored on the iPad.

Does anyone know how we can achieve this - or have any other ideas for how to put a message on the lock screen, which can easily be rolled out to many devices.

Thanks