Would be hard to heat the cassette cog without heating the lock ring as well. It is fairly
unlikely the hub threads would be damaged by overtorquing the lock ring but not impossible.
I would think you could find or rig a long enough extension to a 1/2" 24mm socket to apply
mega torque to the tool such that either it would unscrew the ring or strip the flutes off
before trying the dremel approach. If the flutes strip, you will have the dremel as a back up.
Dremel approach runs the risk of dremeling into the hub threads.