electromechanical torque
I do not know nor found any information on the magnetic torq under full load rejection. However once I experienced a full load rejection on the then still operational Mnt Coffee Hydro in Liberia. If I remember well it took about 5 to 10 seconds before the generator (50 MW) became quiet running again when it tripped from the grid. The whole section was vibrating like there was an earthquake. Then the govener stept in and started to close the wicket gates.

In the link below you can find some discussion concerning your request. I believe there can be some usefull information enclosed

http://control.com/thread/1267218529