Rana pointed out that another way to mitigate seismic motion at in the mode cleaner would be to look at the YAW and PITCH output channels of the WFS sensors that control the angular alignment of the mode cleaner.
I downloaded 45 mins of data from the following two channels:
And did some quick offline Wiener filtering with no preweighting, the results are shown in the PSD's below,
I'm quite surprised at the Wiener subtraction obtained for the YAW signal, it required no preweighting and there is about an order of magnitude improvement in our region of interest, 1-3 Hz. The PIT channel didn't do so bad either.