How to proportion hydraulic flow using potentiometer and MOSFET
It is not unknown technology to simply switch the hydraulic solenoid valve on and off in full switching manner. If the drive waveform is a variable duty cycle PWM signal at the proper frequency you can thus cause the valve assembly to float at a controlled position. 

Note that there are certain types of hydraulic valves that are designed for this type of operation as opposed to others where it would not work at all. So make sure to check this detail with your valve supplier first.