I recently saw a guy on Youtube who made his entire terrain magnetic by adding about 50% iron powder to his paints, plaster and texture pastes. Apparently this does not impact the structure of the pieces (much: it must to some degree, obviously).
Now his work is mostly smallish magnetized tiles for RPG settings, but the idea has absolute merit for wargaming tables. My only worry is that this will increase the weight of our tables significantly

Seriously though; he is able to place minis with magnetized bases on the top of (plaster) rocks without them even moving, let alone falling off. I'm seriously contemplating giving his idea a test...