I was looking at my panel the other day and it looked to me that left and right were part of the trackball and went straight out to the USB with it. The center - well all of them - can be wired this way, too, but on mine they wired it to another button or something. (How often is someone going to use the mouse wheel on a panel?...actually, cos I use Mame32, I would...but I haven't changed it over yet.)
So, I'm wondering why you wired them to somewhere else?