I've added an appendix to the GyroDoc describing the reference cavity stabilization idea. It will need to be fleshed out some more once we've revised the section on displacement noise.

It turns out we would need a length stability of ~6.5 x 10^-14 m/rHz from around 0.01 - 1 Hz with a 20-cm reference cavity. I haven't found a low-frequency spectrum of the PSL refcav, but I know who might have one...