While I was operations manager for Now Audio Video stores I was often called upon to degauss televisions that people had put on speakers or generally magnetized. I am all too familiar with this problem. I also routinely serviced speakers that were so called sheilded units. I can tell you that except for Dynaudio and Morel, all other loudspeakers that I have seen with video shielding had nothing more than a metal cup or a ring magnet fitted on the back of the motor. Both have air holes for motor venting. You can obtain rather large ring magnets from scientific hobby sources. You can also seperate old speaker motors of appropriate size, and steal the magnet. Glue it on the back of your speaker magnet in reverse polarity, and thats it. You will need strong hands and good clamps however. Any magnetically conductive metal covering of sufficient thickness will work as well. Lots of metal objects can be found in hardware and farm supply stores that are ferrous and of good size and shape for the job, you might drill a hole for venting in the middle. This doesn't remove all of the stray field but it will reduce it to "negligible" levels. Since a few seconds of exposure will not usually harm a TV set, you could try your efforts by moving the magnet near the TV to see if you have done it. This may seem a little silly, but you asked for video friendly Pi's. I will bet you will not find too many shielded pro drivers, so you may have to do it yourself.

good luck with this,
Thomas