Observations of the ultra-high energy cosmic ray (UHECR) composition over the last years have unveiled the trend of a flux that is dominated by protons around the ankle and becomes progressively heavier towards higher energies. However, a subdominant proton contribution at the highest energies cannot be ruled out at present. Compelled by the associated, possibly substantial, flux of cosmogenic neutrinos and gamma rays, and the potential for charged particle astronomy with UHE protons, we investigate this scenario by numerical simulation of two independent populations of extragalactic sources of heavy and purely-proton cosmic rays. A fit of the source parameters to the UHECR spectrum and composition observed by the Pierre Auger Collaboration reveals that a proton contribution of up to $15\%$ is possible at the highest energies. The cosmogenic multimessenger signal of this subdominant proton flux exceeds the signal derived from the heavier cosmic rays for most favoured source configurations if the maximum energy of the additional protons is large. In addition, we anticipate a significant flux of neutrinos with energy above the EeV level as a distinguishing feature of this model.