The question isn't clear. At first sight, you try to reduce capacitance of analog traces against it's related ground. If so, that's nothing that can be achieved by a shield.
Besides smaller traces, the most important means to reduce capacitance is using thicker substrates/prepregs. Some low Er substrates can be used in hybrid multilayer designs with FR4, but I doubt it's a realistic option for a 20 layer stackup.
I don't get the purpose of the "grid instead of solid plane" point. Either the grid will be dense enough to keep the ground plane shielding effect, then the capacitance won't be reduced. Or the ground plane isn't acting as a real ground plane.
Having 4 analog layers without intermediate ground planes means a lot of mutual coupling.
For low frequencíes (e.g. < 10 MHz) active driven shields may be an option to reduce effective capacitance.