I doubt very much that even a strong magnet, placed on the outside of a filter, will create much of a magnetic field inside the can, anyway. As an experiment, you might try putting one on the outside of a tin can and then touch the inside with a screwdriver to see if there is any field there. Bet you'll hardly feel anythng.
And, yes, as I started to read this thread, I also wondered why anyone would do this, even if it does work.
Oh, and thanks for the link to that "Amazing Magnets" site. It may help with other things I'm doing.