Hi,
I think you will find that although the HUM is at mains frequency, it will not always be there, of consistent amplitude to give you a reliable clock pulse.
If the source is a machine or electrically noisy device, you may find that harmonics and noise may contaminate the waveform.
Unless you have the hum coming directly from a known consistent source.
If you have the time go for it, worth the experiment.
Tom... 