Recent work suggested that head-to-head and tail-to-tail domain walls could be induced to form in ferroelectric superlattices by introducing compensating “delta doping” layers via chemical substitution in specified atomic planes [Phys. Rev. B 73, 020103(R) (2006)]. Here we investigate a variation in this approach in which superlattices are formed of alternately stacked groups of II-IV and I-V perovskite layers, and the “polar discontinuity” at the II-IV/I-V interface effectively provides the delta-doping layer. Using first-principles calculations on SrTiO$_3$/KNbO$_3$ as a model system, we show that this strategy allows for the growth of a superlattice with stable polarized regions and large polarization discontinuities at the internal interfaces. We also generalize a Wannier-based definition of layer polarizations in perovskite superlattices [Phys. Rev. Lett. 97, 107602 (2006)] to the case in which some (e.g., KO or NbO$_2$) layers are non-neutral and apply this method to quantify the local variations in polarization in the proposed SrTiO$_3$/KNbO$_3$ superlattice system.