Stretching a spring a certain distance will create a certain tension. If the spring is linear, F = kx, where F is the force, x is the displacement and k is the spring constant.
Some form of eddy-current clutch could also provide what you want. Even just spinning a ring of magnets near an aluminium plate will create torque.
All of these methods will require characterisation/calibration over full range of loads and temperature. So you'll have to use a strain gauge at the factory for each device, rather than including the gauge in the end product.
Do you have a picture/diagram of the device you're selling?