Sensorless temperature compensation

For a motor without a temperature sensor installed, would it be possible to measure/estimate the motor temperature based on the resistance of the motor coils? Would it be possible to measure that while the motor is running, or only when still? Has anyone ever done this with a VESC? 

I have a sealed motor, so installing a NTC on the coils would be difficult. The VESC runs the motor very well at first, but struggles to start correctly 100% of the time after the windings get hot.