The short answer that comes to mind is this :
a. measure volts on both sides of L
b. while V higher on left side (incoming) of L than right side (i.e. L is ready/going to conduct/charge the C) AND Q1/Q2 is off, turn on Q3 (with a slight delay to prevent possible shoot-through), and ensuring rapid turn-off when the difference is 0 or close to 0.
My thinking is that Q3 is then doing nothing more or less than emulating the Schottky diode in parallerl with it.
If this is right then making it work with a Arduino ProMini at high frequency (say 500 KHz) could be tricky regarding the internal ADC response.
Two (or more) questions in one go!
Your opinion, please.