I used to use ice cubes on my 27t motors in TC last year before going to brushless. It really helped the motor when running at six or seven minute mains! I have not done this with a brushless motor, but I do not think it would hurt it at all unless you got moisture back in the hall sensors for the pickups. Most do not do it to run a taller gearig either, actually it will keep things cooler for the first minute or two while racing, I could not believe the difference in the motor temps after six minutes.