All in all it seems to me that maybe the best scheme will be to figure out how the sensors work and then build youself a new sensor interface and microcontroller capability to take readings and do your own things with the results.

These days the common and ordinary digital scales, which are seen at times as low as $25 most assuredly use a single dedicated chip such as Dennis has suggested. These chips also connect directly to the LCD glass. Conceptually it may seem possible to connect circuitry directly to the pins that drive the LCD glass but I suspect that this is more difficult in practice than one may think. To save pins on the chip the display digits are usually multiplexed and you can see some odd multi-level voltage signals on these pins which are not usually easy to convert right back to logic levels. LCD glass drivers are also typically designed with little drive current capability since the LCD itself is very high impedance. These scales operate on small batteries and so the special chip will have its drivers designed to be as high of impedance as possible. This will make connecting other circuits here require that the wires be very short. In essance if you elect to glom onto the LCD display lines on the chip you will have to put all your interface circuitry right inside the scale and run your own auxilliary power down to it.

Michael Karas