I would think it possible that a photoelectric effect might occur in proton conductors (https://en.wikipedia.org/wiki/Proton_conductor) where the protons are not tightly bound, but free to move around. Another situation where they are relatively free to move around is in certain hydrated metals, particularly palladium. If the loading factor of hydrogen is high enough in a palladium lattice, perhaps a photoelectric-like effect for protons can be observed at the surface. I suppose that the electrons are still the primary current carriers in this case, and so hydrated metals are not considered "proton conductors", despite the high mobility of the protons. If I were to venture a guess, it would be that the work function for the mobile protons would be about the same as for the electrons, and so the ratio of the number of protons emitted to the number of electrons emitted photo-electrically would just be proportional to the ratio of the density of states for the two particles at the surface.

I would think it is possible that a photoelectric effect might occur in proton conductors where the protons are not tightly bound, but free to move around.

Another situation where they are relatively free to move around is in certain hydrated metals, particularly palladium. If the loading factor of hydrogen is high enough in a palladium lattice, perhaps a photoelectric-like effect for protons can be observed at the surface. I suppose that the electrons are still the primary current carriers in this case, and so hydrated metals are not considered "proton conductors", despite the high mobility of the protons. If I were to venture a guess, it would be that the work function for the mobile protons would be about the same as for the electrons, and so the ratio of the number of protons emitted to the number of electrons emitted photo-electrically would just be proportional to the ratio of the density of states for the two particles at the surface.