We have observed a large coupling between the excitonic and photonic modes of an AlAs/AlGaAs microcavity filled with an 84 Å/20 Å GaAs/AlGaAs superlattice. Reflectivity measurements on the coupled cavity-superlattice system in the presence of a moderate electric field yielded Rabi splittings of 8.5 meV at $T = 293$ K and 9.5 meV at $T = 238$ K. This splitting is almost 50% larger than that found in comparable microcavities with quantum wells placed at the antinodes only. We explain the enhancement by the larger density of optical absorbers in the superlattice, combined with the quasi-two-dimensional binding energy of field-localized excitons.