If you only need to power the device when the motor is off, and "off" means it is isolated from the supply at the supply end (i.e. not going though a variable speed drive or similar) , then I'd be looking at injecting a high frequency ( maybe 10-20khz) signal between a phase and PE.
Couple this at each end with a fairly low value cap ( ? 1-10nf?), rated at the full supply voltage, so when the motor is energised, you won't get much current at mains frequency, so a simple zener could be used for protection. The low cap value should also avoid any issues with earth-leakage tripping etc.
You could send low-speed data back by modulating the power draw of your device, so you only need one cap at each end.
If you need to send data to the device, you could use Manchester code on the driving frequency.
To drive the line, maybe use a small audio power amplifier, and detect the modulation by measuring changes in the DC power draw of this amp.