(Is it possible to rig some form of built in scale/weighing mechanism to an aircraft undercarriage? Has this been done before or attempted?)
Even if it were feasible it would very soon be discarded or stripped-out. Anything that is seemingly uneccessary on a cargo aircraft is stripped-out to increase payload thus increase revenue.
Mentioning no names but some cargo airlines are reputed for knowingly operating several tons overweight and on a regular basis. If such a reputation is earned then shippers and/or ground staff may exploit it.