Low voltage control of 75V piezo actuator
There's 2 ways I can see doing this.


Start with an adjustable switching regulator (probably a boost regulator), then modify it so that it can be controlled by your PI. For example, you may be able to do something as simple as replace a potentiometer with a digital potentiometer. You also may be able to find a supply that can be digitally controlled.
Basically do the same thing except design the whole thing yourself. Design a boost converter, then either find the voltage reference and replace that with a DAC, or do the digital potentiometer thing.


Edit: Ok since nobody else has answered, here are some more complicated, higher speed options.


Use a DAC and a high voltage op amp like the OPA445AUG4. Use a resistor to limit the current into the piezo element. 
To get even faster you will need to increase the current into the piezo. I would do this by buffering the op-amp with mosfets or BJTs, like this:

If you use mosfets, try to get ones with a small gate charge. Since that will be the limiting factor on your speed.