Technically you could glue the gut the stops and glue the pot in the center position.
That's all you really need.
Step 1
Remove case of servo to expose the gear train, motor, and potentiometer. This is accomplished by removing the screws on the back of the servo case and separating the top and bottom.
Step 2
File or cut off the nub on the underside of the outpout gear that prevents full rotation. This may mean removing some of the gears so remeber how you put them in.
Step 3
Remove the pot (or find the center position on the potand glue it there) and replace it with two 2.7K ohm 1 percent tolerance resistors. This fools the servo to make it think it's at center.
Step 4
Ressemble.
There generic hacking directions. Of course who knows if it's legal or not to hack a servo.