I think its a great idea. I never used the magnet on the filter, but have always used the drain plug with the magnet slightly protruding from it. When you change the oil, you can see what you have picked up on the magnet. I just wipe the debris off and make sure it is clean before reinstalling. With the filter magnet, you really cant see what you have collected, but you can be sure you got something.
You can buy some small rare earth disc magnets and put them on your filter and even the head of your oil drain plug. A good neodymium magnet will be strong enough to magnetize the threaded end of your drain plug.