Just found this on Youtube.

http://www.youtube.com/watch?v=K7uIyPhdb2I

So FFXI can work on three displays.

The only problem is that without the correct aspect ratio you're stuck arranging the monitors into a very large 16:9 display.

So what we need to make this work properly is:

Custom aspect ratio.

Some sort of "only draw the overlay X width" setting in order to restrict the UI to the center screen. So if you were using 3x1440x900 monitors you'd tell it to only draw the overlay 1440 pixels wide.

Neither of these things seem like they would be very difficult to implement in FFXI.