Magnets instead of shear pins

No, but I did know of a guy who used electromagnets of reversed polarity in lieu of an ejection charge. He coiled wires around two horshoe-shaped magnets, one affixed to the upper airframe, one attached to a recess in the nosecone shoulder. In the payload bay he had a nine-volt battery to power it up. The whole thing ran through a two event timer. It was timed to actuate just after apogee, which worked pretty well. That way, the magnets simply had to "help gravity". The really great thing was, the whole setup culd have fit into a Quest SuperBird, but if I remember right, he had it in an old Estes Honest John with an Aerotech F, I think.

Nope. A nosecone won't fall off due to gravity at any point in the flight. Gravity is working on all parts of the rocket uniformly. (Drag differential may cause the nosecone to come off, but that's a horse of a different color.)

Said another way: Those magnets will do the same work whether before or after apogee.