How to hack a body scale to be used as digital weight measure
Sounds not too difficult to do what you are proposing, most digital scales use 1, 2 or 4 strain gauges in a Wheatstone bridge configuration to form a load cell.
This load cell will output a small differential signal which is read by an Instrumentation Amplifier (InAmp) to amplify and output a single ended signal that can be easily read by an ADC.   

So if you have a digital scale, it's just a question of hacking it and tapping this signal to be read by your PIC, Arduino, etc, to be used as you like (send to PC, display...)
As far as I'm aware the R-Pi doesn't have analog inputs available as "standard", and it sounds like complete overkill for this anyway (unless you are using the R-Pi in place of a PC or something)