If you are meaning the driver gets off the forklift while in racking and raises or lowers the hydraulics while not on the seat, how about a seat switch and solenoids on the hydraulics? If the driver gets off the seat, the hydraulics wouldn't function. Then you could use the original sized load backrest.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.