Is there a simple, analytical formula for the lunar phase brightness curve?
The magnitude of the Moon without the opposition surge is given by a simple equation:
$m = -12.73 + 1.49 \cdot |\psi| + 0.043 \cdot \psi^4,$
where $\psi$ is the phase angle in radians (Allen 1976).  This can be converted to flux using $m \propto -2.5 \cdot ^{10}\!\log F$,
where $^{10}\!\log$ denotes the 10-base logarithm (as opposed to e.g. the natural log).
For the increase in flux due to the variable distance and opposition surge, I multiply F with the factor
$\left(\frac{\Delta_0}{\Delta}\right)^2  \cdot  \max\left(1, 1.35 - 2.865 \cdot |\psi| \right),$
where $\Delta_0$ and $\Delta$ are the mean and current distance to the Moon, respectively.  However, I can't find a reference for the last term at the moment, and I can't remember where I obtained it.  I'll post that information if I find it.