There is more to be reckoned with.
First of all: centre of gravity. A Rifleman arm will field most of its mass close to the rotational axis, thereby making it far more easy to use magnets. A Marauder? Not so much... Most of its weight is far away from the magnet in the 'hands' thus creating a big lever. As a result the Marauder arms will point downwards most of the time. This can be mitigated by using stronger magnets (on both sides (torso/arm)), but it will still be a matter of magnet size, respectively friction und high load pressure. Using a pin in excentric mount position plus indentations on the other side to stop the arms from dropping would prevent some of the positioning problems but recreate the problem that lead the TS to ask.
I have mounted magnetized arms on several minis, mostly those with additional weapons like the Shadow Hawk -7CS/-7M, the Griffin from the Wolf&Blake pack or the Rifleman -6X/-7M. While the Rifleman arms are good as they are, the Griffin arms are already a pita as they are heavier and i was unable to mount the same large magnets as in the Rifleman. The Shadow Hawk is somewhat like the Griffin, but its arms are lighter, thus not as problematic.
Other positions to mount magnets can be far more easy. I own an old sculpt Vulture with its vulnerable arms. In order to prevent what you wrote i mounted it with magnets in torso and hip piece. The whole torso can separate before the arms or its lasers will do.
As a sidenote and tip: as you might know the hip piece is quite shallow. In order to prevent a puncture while drilling the hole for the magnet i inverted the position of the used pin and hole system. Before the hole was in the hip piece and the post in the torso, afterwards the magnet in the hip piece acted as pole and the torso mounted magnet was sunk really deep, thereby acting as rotational axis, too.
As a last example i mounted my Deimos (one of the early production not by IWM) with magnets in torso an hip piece. It was a necessity as this thing is huge - too big to be transported in my jig to games. Transporting it in two pieces settles the problem.
