[garba_dlm]

would it be possible to induce the vibration of the metal piece (assuming it's magnetic) without nails?

using very rapid magnetic oscillations (like a speaker??)?

I think the electronics would be different, but is it possible?

[throwaway-42808]

probably!

You might like the Magnetic Resonator Piano: http://instrumentslab.org/research/mrp.html

[perilunar]

Cool device. The electronics for "continuous note shaping" would be a lot more complicated than just 'hitting' a note, which could be as simple as a single pull and release (probably not very loud without a much stronger electromagnet).

You could send a plain sine wave matching the frequency of the bar above it, but at that point you've just invented a single frequency speaker.