Don't know of any reason why it wouldn't work. However ... I'm guessing the wire vertical is NOT grounded at the base of it, but the tower is? That may have some effect on it. Maybe if you could test further up the tower? AM broadcast towers can be either series fed, where the base of the tower is insulated from ground and the tower is fed against ground at the base, or shunt fed, where the base of the tower is grounded and the tower is fed with a wire connected some distance up the tower from ground. Consider how this may affect your ability to measure the resonant frequency. And does the tower have guy wires? If so, I assume they are grounded, shunting to ground any potential resonances, or possibility creating a number of "sub-resonances."
Jim Allyn - N7JA