Over the years many have tried their hand at designing an onboard weight and balance system. All have failed in deriving a system that could be certified for operations. Even if you could prove 10 to the negative 9th that a failure of the system would not cause a crash you are still stuck with designing a system that even one failure is unacceptable.
Boeing offers a weight and balance system on the 747-400. They also offer a dual backup system. The single system is standard on the freighter with the backups system being an option.
To date, only one 744 operator has bought the backup system as an option. I know from speaking with them that neither of the systems will ever replace the loadmaster.
Most PAX 744 operators that chose to pay for the WBS from Boeing have deactivated the system. Anything sensative will be knocked out of calibration by hard landings and dirt will end up covering any type of lazer or infrared calibration system.
It would be nice if you can get a system to work, but I think you will find it would be cheaper and easier to build a massive in ground scale that weighed the aircraft like they do at truck stops.