Any way to shield a non-sheilded center channel ?

The metal plate must be a material that can be magnetized. That way the magnetic lines of flux from the speaker are concentrated in the metal and they are directed away from the TV. Lead is nonmagnetic and will not work. As mentioned, mu metal is an excellent choice. Do an internet search for sources. Many (most?) "shielded" speakers are not shielded at all. They have another equal strength magnet behind the speaker magnet that concentrates the lines of flux inside the cabinet.

On first reading I missed the comment about the ground above. Connecting the metal plate to an electrical ground like an outlet cover screw will have no effect. Magnetic flux lines cannot be grounded, they must be guided away from the TV via some magnetic material.

The other option not mentioned is distance. The flux lines spread out and exert less influence as the speaker is moved away from the TV. However, the distance that it takes to achieve good results may not be acceptable in your setup.

Sean, continuing on in ignorance is not too bad. I've been doing it for years. I agree that installing an extra magnet in the your speakers is probably a bad idea. I just pointed out this design technique in the hope of clearing up some of the confusion about shielding.

As far as using lead to shield against RF, your defintion of "concentrated and controlled magnetic waves that "float" at high frequencies" leaves out the electric field that is also part of the wave. RF, x-rays and the like are electromagnetic waves of various frequencies. They are made of alternating electric and magnetic fields that vary in amplitude as they travel through space. Lead will block RF, as will copper and aluminum and any other metal that is a good electrical conductor.

This is a different animal than a magnetic field like that from a speaker magnet that is not varying. It has no frequency. It is a constant pull against the opposite pole unless you move the magnet. Kind of like gravity exerts a constant pull except magnets have two poles, north and south, and gravity is a monopole. This magnetic force is usually desribed in terms of lines of force (flux) that go from the north to south pole. Remember the metal shavings and magnet experiment and how the shavings line up. The flux lines cannot be blocked or broken, only redirected by placing a magnetic material in the field. The flux lines will follow the path of least resistance, which is the magnetic material. So placing a magnetic material between the speaker and the TV will concentrate the lines in the metal and they will be weaker on the other side.

BTW, lead is also an excellent shield against Superman's X-ray vision.