  We develop a method to constrain non-isotropic features of Cosmic Microwave
Background (CMB) polarization, of a type expected to arise in some models
describing quantum gravity effects on light propagation. We describe the
expected signatures of this kind of anomalous light propagation on CMB photons,
showing that it will produce a non-isotropic birefringence effect, i.e. a
rotation of the CMB polarization direction whose observed amount depends in a
peculiar way on the observation direction. We also show that the sensitivity
levels expected for CMB polarization studies by the \emph{Planck} satellite are
sufficient for testing these effects if, as assumed in the quantum-gravity
literature, their magnitude is set by the minute Planck length.
