This should be simple but cannot get my head around it. Want to create a bootable usb drive (FreeDos, Syslinux GrubforDos) really don't care. All I need is it to boot and display a simple message and enter to quit and reboot.
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I'd personally use Syslinux / ISOLinux with a non existent default menu item with pressing Ctrl Alt Del to reboot the box. The title on the non existent item could be your message.
As long as you don't specify a timeout in the menu config it should do about what you want.