How to achieve feedback in regenerative braking?
A boost converter isn't really the best way to think about what is happening during regeneration. It's true that inductance boosts the voltage in both cases, but the topology is different.  There is no voltage feedback because you aren't controlling the voltage; it's fixed by the supply voltage.  You can control the current however.
To get the motor to brake, you apply reverse torque by reversing the current in the motor phases.  Exactly how to accomplish this depends on the switching scheme.  For example, if you are alternating upper and lower switches in all active half-bridges, you don't need to do anything special.  In this case simply reducing the duty cycle to lower the average applied voltage below the back emf reverses the phase current, and power will flow from the motor back to the supply.  Be sure to have a supply voltage monitoring circuit that can shut off the bridge if the voltage gets too high because the supply can't absorb all the power.