I would say that there's too many variables out there (different copiers, printers etc.) to have it work consistently.
If the client really wants something similar I'd suggest talking to the printing company and see if they have any inks similar to what they use checks and pass the 'void' graphics on to them to apply it for the client. I'm guessing the printing costs would go up for that service. If there's any other solution I don't have it. Good luck to you! I don't believe the inks are what allows the void pantograph to work successfully.
While there are inks with special security features (color-shifting properties, for example), the void pantograph is generally created using prepress software and requires specific line screens.
