TDB said:
Isn't the torque arm usually attached directly to the axle? Axle can't rotate easily when clamped into the frame.
Single speed coaster brake hubs have their torque arms attached to the left side axle cone. They can't move back and forth without loosening the hub and causing problems.
Multi-speed coaster brake hubs usually have torque arms that are captive but not rotationally locked to anything other than the brake expander cone. This could hypothetically be used to actuate something, if as MadRhino suggests you provide a spring return of some kind. Braking force on the torque arm can be very high, so it must be firmly retained.
I once had a plan to use the brake arm of a two-speed automatic coaster brake hub to actuate a cable running to the front brake. I never did work out a good way to extend, travel limit, and spring return the arm for this application, but I guess I could return to the project one day.