So far as I know, the answer to this is no. Radioactive decay is actually a complicated process. For individual particles such as neutrons and pi mesons, it involves the 'weak nuclear force' which is not modifiable by any electromagnetic means. The radioactive decay of heavy nuclei usually results in the quantum mechanical 'tunneling' of alpha particles out of the nucleus of the radioactive atom. The tunneling rate is determined by the height of the nuclear potential 'well' which is dominated by the strength of the strong nuclear force which at these scales is 100 times stronger than the electromagnetic repulsion between the nucleus and the alpha particle. In principle, if you could modify the electromagnetic part of the nuclear potential, you could change the decay rate by a small fraction of a percent. The only way to do this, however, is to actually alter the distribution and number of charges in the nucleus. Since the nucleus is much like a drop of water, one could imagine applying a force to the nucleus to elongate it so that the nuclear potential well is lower in some directions than in others. The alpha particles would then tunnel out of the nucleus faster in some directions causing the decay rate to be anisotropic.
I don't know if this would work, but any simple electromagnetic interaction would be ineffective.