jremington - you're right of course.

1/ modelling a piezo as a series capacitor and resistor isn't very accurate , but it's a reasonable starting approximation. A better model will not be linear, but will vary strongly with excitation voltage.
Can't be bothered.

2/ my crude circuit does not, as you say, provide any voltage gain, but it will provide much more current, which may be useful driving a low impedance series resonant load.The series l-c provides the voltage gain at resonance.

Pwillard : your circuit is a basic boost voltage convertor - but we don't want high voltage dc.

Danielvt. Your implementation of my circuit is inaccurate. Get it right and try again.

1 resistor, two transistors. Omit the 100uF capacitor. Could anything be much easier?

Allan