my magnet was firmly and permanently attached to the battery,
If you're absolutely certain that the magnet can't move in such a way as to short something out, then it's probably OK.
I'll give you an example you can relate to: If you were using a magnet as a spacer at the bottom (negative end) in a CC mod, I'd have no issue with it. It's a top-firing mod and the body of the mod is always connected to the negative end of the battery. A magnet down there can't possible cause a short.
However, I can't really picture how that would be the case in a stacked mech, particularly a bottom-firing one. But again, I'd want exact details before I said, "
That's OK", or "
You're risking blowing your face off with that, and here's why." Without such details, I lean heavily toward: "
Don't do it!"