I've always thought this would be awesome and haven't seen it done yet.
This project is in its infancy, but I think I've gotten far enough to prove the concept can work.
This past weekend, I picked up a Gauntlet Legends CP from a showcase cab:

I thought it would be pretty cool to put a monitor in the little marquee area and have it display instructions, etc.
Even more cool would be to use a touchscreen and be able to flip through move lists or simulate extra buttons.
In another thread, the program Touchbuddy was mentioned as a possible way to add a custom keyboard or controls without windows losing focus.
It is used primarily by flight sim enthusiasts. I haven't seen it used on a cab yet. (I'm sure there's a flight sim/arcade guy out there somewhere)
http://www.touch-buddy.com/forums/I downloaded the program and accompanying videos. It is a LOT easier to use than I expected. Pretty much just choose a picture to use as a button, drag it where you want it, and assign a key or macro to it. What caught my eye was that it can cycle through different pictures of the same button, so a toggle switch can appear up or down, or a light switches between red and green. I noticed that a guy on youtube had a dial that cycled between 3 positions. I wondered if you could use this feature to cycle through move list cards for each character in a fighting game, so I did a test with a few SFIV images. I borrowed an ELO touchscreen from one of my Megatouch machines, but I couldn't get the serial drivers working with Vista64, so this test was just done on a regular screen with a mouse.
It looks like it will work! I only tried a half dozen images, but it didn't look like there was any kind of limit to how many you can cycle through.
You can't go backwards, but the images will keep looping around. I haven't tried it with the game running, but Touchbuddy isn't supposed to take Windows focus away from the other programs that are running.
What I envision is a move list on each side of the touchscreen, so each player can select their character.
Maybe separate them by a picture of the original button layout & basic controls if there is enough space.
Each game would have it's own Touchbuddy profile that would be launched via command line when the emulator is launched.
You can also have tabs across the bottom and Touchbuddy can display pdf files or webpages on other tabs.
Imagine a picture of an Atari 2600 where you touch the original controls to reset, change difficulty, etc.

It's a daunting task to make a move card for every character in every game, or even most games that I'd want.
I have a big folder full of every move list scan I could find on the internet, but chopping up those doesn't really yield the results I'd like.
One problem is that unless they are all aligned exactly, the card appears to jump around as they cycle through.
Some of them are spread out horizontally, some are more vertical.
I'd like to have some type of template, where I could just drop in blocks for joystick movements and button presses.
Anyone have any ideas about the best way to go about this?
I was thinking if enough people were interested and there was a general consensus about the size and design of
the move cards, maybe it could become a community project, with many people contributing.
You wouldn't have to use a touchscreen for the cards to be useful.
Once the pics are made, they could be accessed a lot of different ways.
I always thought it would make a cool Android app.
Another thing I thought about was.....
All this information already exists in COMMAND.DAT
http://home.comcast.net/~plotor/Would it be possible to write a program that uses the command.dat file to automate part of this process?
The character names, move names, and required movements are all there. It would be awesome if it were
possible to use that info, match it to a pic, and spit out a nice looking moves card without spending hours and hours making each one.
This is way out of the range of my capabilities, but maybe there is someone out there who likes the idea and has the talent to do it.
That's all I have so far. I'm getting busy at work and have other projects.
Progress on this is going to be very sloooow if it's left up to me.
Here's a video of Touchbuddy working: