Players of stringed or tuned-percussion instruments will be familiar with the concept of sympathetic resonance: if I play a C on the piano with the damper lifted, all the other Cs (and, to a lesser extent, the Gs, and so on) will resonate along with it -- and continue to do so even if I damp the original C.
It would be possible to construct a "tuned reverb" system, based on the same principle as a plate reverb, but seeking to maximise rather than minimise tuned resonance. (Imagine clamping a speaker and contact microphone to a piano frame.) This could be used to provide a resonant backing behind a solo instrument -- although the player would probably want a damper pedal to stop the resonance when changing chords, in the same way a pianist does. I've seen this done to great effect when using a MIDI-equipped guitar and a synthesiser, but this would work with any sound, including the human voice.
Of course, you probably wouldn't build this in real life; you'd simulate it using something like Pd...