Has lunar opposition surge ever been observed from Earth? From Earth orbit? This thorough answer to How long does lunar opposition surge last? Are there measurements of the full Moon getting suddenly brighter? details observations of lunar opposition surge by the Clementine and Lunar Reconnaissance orbiter.
But we don't have to go to lunar orbit to see the sunlit Moon, we can observe the lunar disk right here on Earth.
Question: Has lunar opposition surge ever been observed from Earth? From Earth orbit?
While I couldn't find any quantitative information about views from Earth or orbit down to zero phase angle (especially since views direct from Earth are limited in phase angle due to Eclipse), this website gives an unreferenced data point suggesting Apollo astronauts observed that a zero-phase full moon is approximately 30% (0.2 magnitudes) brighter than we can ever observe from Earth.
This site cites a value stating that the moon's brightness at zero phase angle may be as much as 14x brighter than an ideal diffuse reflecting disk, with 40% of the increase within the last 5˚ of phase angle (which cannot be observed from Earth).
Finally, there is one data set I have found which seems to show the opposition surge effect, but as noted, it is not what I would consider complete, and only has a few data points outside of Earth's viewing capability. That said, the source does not provide a value for maximum reflectance based on their model.

Robotic Lunar Observatory data, Reflectance vs Phase Angle