Joe,
The hard part isn't doing the display; I could do that in 20 minutes and be done with it. The hard part, if this is to be a public release and not just a one-off, is not requiring each cab owner to create an image of his control panel for every single game! If we can create some sort of workable format for this that reads your controls from MAME's config files, we should be able to build up a library of instruction files, similar to the way artwork files are handled now. Once we can make that work, it can be displayed in frontends, on separate screens, on separate computers, whatever.
--Chris