Strong coupling with single molecules in plasmonic picocavities has emerged as a resource for room-temperature quantum control with nanoscale light. Tip-based nanoprobes can measure the local dynamics of individual molecular picocavities, but the overhead associated with sampling an inhomogeneous picocavity distribution can be challenging for scalability. We propose a macroscopic approach in which an ensemble of molecular picocavities acts as a nonlinear plasmonic metamaterial. Using a quantum optics perspective, we study theoretical performance limits for optical cross-phase modulation in the system, taking into account realistic distributions of picocavity volumes and molecular transition frequencies. The medium nonlinearity is mediated by local strong coupling with the lowest vibronic emission sideband of individual organic chomophores. The local vacua change the refractive index of the medium at the frequency of a weak probe field $omega_p$, set to drive the bare zero-phonon absorption band. Refractive index variations $Delta n/n$ of a few percent, relative to a molecule-free scenario, are feasible with dilute ensembles. The probe phase evolution can be switched off in the presence of a signal field at a higher frequency $omega_s$, for intensities as low as 10 kW/cm$^2$. The mechanism for optical switching involves a novel ($omega_p+omega_s$) two-photon absorption channel, assisted by local vacuum fields. Our work paves the way for future studies of plasmonic metamaterials that exploit strong light-molecule interactions, for applications in optical state preparation and control.
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