Nickel sheathing would easily do the job.
The earth has a natural magnetic field of 500 mG. At surface level it's less than 1 mG. In a hybrid the level is amplified to 300mG (about 150 times more than an ICE equipped vehicle and in the case of the Prius this is right between the driver's legs.) I am a strong supporter of hybrid technology and I'll gladly pay the extra coin to reduce the EMF either in production or aftermarket. Give us a solution automakers. Some will buy, some won't.
Hey, remember when doctors used to advertise/promote smoking? It took us a while to figure the threat out. The same may be true of concentrated levels of EMF. In the meantime, I'll go for the protection.