Looks pretty easy for anyone with basic machining capability to add to an existing neck plate. The biggest challenge would be to avoid screwing up the chrome plating on the plate itself. The hole you drilled in the neck plate could be tapped to take a long set screw, and it would be easy to inset a metal slug into the base of the neck for the set screw to press against.
OTOH, how often would you ever need to make an adjustment? Not very.