Honestly, if the knocker is a real concern, the other option would be to simply get a small motor or aquarium pump (if you use a motor, you'd want to get a brushless DC motor, and use electrical tape and tape a fishing weight to the shaft, so it's unbalanced), and then when you turn it on, it will quietly hum away (you can get real quiet ones). Tape or strap it to the side of the powder measure, and it will vibrate the kernels into the measure. You couldn't just go "slam slam slam slam" really quick with it, but realistically, if you're using the knocker, then you're not going super super fast anyways.
Basically you'd measure out one charge, then move the handle back to the up position, and while you move to the next case, the motor/pump would vibrate the powder into the measure chamber, then you pull the handle back down and dispense, and then move the handle back up before you move to the next cartridge.
That second or three in between cartridges would be more than enough for the motor to vibrate the powder pretty consistently. I would actually guess it might even be more consistent than using the knocker, because you might not be perfectly consistent with the knocker