Switching converter current mode control - load disconnect problem
If instead of using a MOSFET and diode non-synchronous buck regime you chose a synchronous (2 MOSFET) scheme you would get what you want without the hassle of a control loop.

A synchronous buck converter power stage is this: -



Either Q1 is on or Q2 is on - this translates to a rapidly alternating changeover contact producing a square wave of varying duty cycle (duty controlled by possibly a MCU).

The average value of the output square wave is Vin x duty cycle. This output voltage is low pass filtered by the inductor and capacitor so, there is need for a closed loop control should the load become disconnected. 

The usual caveat of a sync buck converter applies; constant output voltage with load is only guaranteed if the input supply voltage (Vin) remains constant. However a non-sync buck converter cannot deal with input voltage variations either without a feedback loop. It's easier for a sync buck because duty cycle can be determined by an ADC measuring the input voltage and making a simple correction to duty cycle.