The only reef safe magnets that I know of are the rubber-coated Red Sea ReefWave magnets - which probably wouldn't work for your purpose. Everything else typically attaches to the outside of the tank. While many of the rare earth magnets are coated, I wouldn't necessarily trust them for any length of time in a saltwater environment.
Having once owned that IM defroster, I'd probably be more inclined to jury rig a mount that you can clip to the side of the brace