How about two scales?
Rapala, Mustad, and Etc. make mechanical tube scales for weighing fish. This kind of scale has a handle that can be hung on a tree or simply just held in your hand. This would be great for weighing a full pack at the trail head. However the scales are usually lbs and I'm not sure how accurate this type can measure ounces. They usually come in 30lbs and 50lbs limits.
For weighing small items in ounces you might try a portion control scale. The type used in kitchens to weigh out food portions. These are table top units, and not something you'd drag to camp (or maybe you would?) This type is also mechanical.
I usually just weigh myself with and without my pack, then subtracting the latter from the former I determine my pack weight (and consider a healthier diet!)
Hope this is a step in the right direction.