The KSM Power Off function displays a simplified HTML page. (Like a single web page) It can contain text and images, but what I found easiest was to have it display a full screen image file produced using Gimp2.
My "poweroff_info.html":
<html>
<head>
<title>kobo menu info</title>
<style type="image/png">
p {
margin-top: 0.5em; }
</style>
</head>
<body>
<img src="/mnt/onboard/.adds/kbmenu/txt/poweroff.png">
</body>
</html>
The poweroff.png I created was sized to 1040x1387 to fit my Aura HD. Notice that I have the "body" contain only the image file, you could build a page with smaller images and text, but I find it easy to create a single image in an image manipulation program. For your use, you could make up a number of signage image files, with descriptive names, and use KOReader's File Manager "Rename" function to change the current named poweroff.png - right on the device.
Luck;
Ken