Nige,
Sensor wires are now very rare on brushless kit
The motor is a passive lump: electrically is is the ESC that does all the control and has to give the motor the electrical food for its nourishment. The ESC senses the motor rotation and timing without any help (it looks at the voltage on the power wires to trigger the pulses)
So the ESC will happily run the motor. without the sensor wire - just insulate the end of it
I am surprised that you say it has a Hall wire - my (fallible) memory tells me that sensor wires were usually 2 extra wires - much thinner than the power wires and with a small connector block fitted.
In case it is of interest to anyone - CDROM motors in their natural habitat (on the circuit board, in a CDrom drive) have a little entourage of hall effect sensors around the edge of the rotor - these are visible as 4-legged chips (about 1.5mm square) and there are often 3 or more of then doing the timing and speed control for the CDROM drive.
I keep thinking that I should make more use of these sensors - I throw them away when I howk the motor off the board
How about a brushless DC motor?
andrew