So, just to be super safe and transparent, I provide my Standard Disclaimer, below . . . .
You omitted "beware of falling rocks". Ha! See ya in court!
I am also concerned about possible polarization issues between a strong external magnet and an internal magnet in the cartridge, you know, attracting versus repelling, and other issues spring to mind in addition to the relative strength of multiple interacting magnetic fields.
Don't be.
You cannot depolarise a permanent magnet this way. Especially during such a short exposure. You may stop worrying.
If you heat it high enough though...
Just do not use an oxy-acetylene torch.