A theory for an additional effect that increases brightness during opposition is that of coherent backscatter. In the case of coherent backscatter, the reflected light is enhanced at narrow angles if the size of the scatterers in the surface of the body is comparable to the wavelength of light and the distance between scattering particles is greater than a wavelength. The increase in brightness is due to the reflected light combining coherently with the emitted light.
Coherent backscatter phenomena have also been observed with radar. In particular, recent observations of Titan at 2.2 cm with Cassini have shown that a strong coherent backscatter effect is required to explain the high albedos at radar wavelengths.
While the word "light" is mentioned, the references refer to essentially monochromatic, longer wavelength radar measurements. Opposition surge is probably due to the simpler, well-understood geometrical effects known as shadow-hiding and retro-reflection or Heiligenschein. You can see slightly brighter ground reflections around the shadows of astronaut's helmets in photos on the moon, for example in this as-yet unanswered question and also in this as-yet unanswered question.
Coherent backscattering using visible light can be demonstrated in the laboratory in situations where one has great control over the particle size and index of refraction contrast, for example single sized (monodisperse) sub-micron transparent plastic spheres suspended in water 1, 2, 3, 4, and with narrow-band light with transparent biological samples 5. At radar wavelengths, rocks and soil particles are not opaque, but instead become dielectric particles, thus the observation of coherent backscattering at radar wavelengths from astronomical bodies.
In light of this insufficiently-answered question's multiple re-bumping by community, I'd like to ask - are there any visible light images of Earth that contain bright sub-solar spots that are generally agreed to have been produced by coherent backscattering?