I understand that this would costs some bucks to do, but I have always wanted to use a widescreen format TV wide enough to stretch across most of the width of the cab.
The way I was thinking about doing it was to de-case the unit, cut a slot in the top of the cabinet and slide the screen into the marquee area. You would then just use the bottom portion of the screen to display your marquee images. Of course, the rest of the screen is still sticking up above the cab, so put it to use! You can display the instruction cards for the game, run arcade related animations, use it to display the name of your cab, whatever. If you do it right, it would look like a narrow LCD sign sitting on top of the cab, while the marquee would look like a marquee, but changeable!
Aside from that, I think the projector Idea is cool if the logistics can be hammered out.

RandyT