I can't speak for interference problems with any expertise but any shielding material will have to be ferrous itself, e.g. a sheet steel box/covering for the Motronic unit. Lead is good for radiation insulation but that's all it's really going to stop. It's all about redirecting the magnetic flux around what you're shielding, can't just block magnetism.
Speculation: With the digital electronics in the unit I don't suppose any permanent damage would occur from a test fitting and while there is a changing magnetic flux when the speaker is operated I don't think it would be enough to affect anything. Enlighten us with results, Duck!