Maybe they could use small home treatment units in Japan to remove radioisotopes of iodine from drinking water there. After all, as wikipedia reports

Activated carbon does adsorb iodine very well and in fact the iodine number (ASTM D28 Standard Method test) is used as an indication of total surface area.

Such approaches would have to be verified, and methods would need some level of standardization, but maybe if they run out of bottled water in Japan, they will need to think about this approach.