I wouldve looked at that but one issue is size of the rover so the electromagnet was a good choice because it can be small enough to fit onto the rover without being much of a hinderance. Thank you for your suggestion thoHow about using suction? Any smooth object would be able to be picked up and release with this method. There is a company that makes a complete kit that is be controlled by a servo (Lynxmotion)
http://www.lynxmotion.com/p-788-vacuum-gripper-kit.aspx
by Aaron Carman
by Jake Hertz
by Jake Hertz
by Jerry Twomey