We consider an effective model of photon interaction with a scalar field corresponding to conformal excitations of the internal space (geometrical moduli/gravexcitons). We demonstrate that this interaction results in a modified dispersion relation for photons, and, consequently, the photon group velocity depends on the energy implying the propagation time-delay effect. We suggest the use of the experimental bounds of the time delay of gamma-ray burst photon propagation as an additional constraint for the gravexciton parameters.