I did a search, and found no topics on this.
I haven't checked this myself, but I've got an inside source that if you check the graphics ROM in Aurail (arcade), you will find some really cool hidden artwork.
There's supposedly a large unused space.
If you find it, post screens, because I'd love to use it in my book in the chapter discussing it. Full credit will go to whoever provides it.
More secrets as I discover them!