Is the effective nuclear charge only defined for valence electrons?
$Z=Z_{eff}+\sigma$, where $\sigma$ is shielding constant calculated according to Slater's rule. Since Slater's rule can be applied to any electron regardless of whether or not it's a valence electron, I think $Z_{eff}$ and $Z$ can be defined for any electron too, $Z$ in particular remaining constant.
The effect of outer electrons not being taken is also explained by Slater's rules empirically.