Magnesium is not stronger, its ligher, and has less impact strenght than aluminium, its only really used for pedals, becase normally the body of the pedal isnt under much stress, all the weight and stress is put on the axle that the pedal is on...

Most seatposts are aluminium. Shadow made a Ti post, but with the rising prices of Ti they discontinued it...

If you want a light seatpost, get a Fit, Odyssey or Macniel(only if you want a new seat too)