Download: http://caplagrobin.deviantart.com/art/Batman-Arkham-City-Superman-Mod-589660960
Put the txt files in a folder named MS in steam\steamapps\common\Batman Arkham City GOTY\Binaries
How to load the meshes:
Go to the Challenge maps, highlight first the default Batman and enter the codes ("LoadPC Playable_Batman | exec ms/superman.txt | exec ms/smpowers.txt") (for the cape), then highlight the 70s Superman and enter the codes again (for the body). Now for the head, go to any challenge map, and keep applying "LoadPC Playable_BruceWayne" before you enter it, and while it's loading, like a mesh code. When it stops loading, go back to the menu, apply the Superman codes again. Now, go to free roam and keep applying the codes in the loading screen. Press these buttons for the super powers:
Superflight off: Numpad 7
Superflight on: Numpad 8
Telescopic Vision off: Numpad 4
Telescopic Vision on: Numpad 5
Invincibility off: Numpad 1
Invincibility on: Numpad 2
Floating off: Numpad 6
Floating on: Numpad 9
And I know about the gap in the neck, it's unfixable.