I've checked that the 2pin lemo connector that was run some time ago from the LSC rack to the MC board does indeed transmit signals. To try and evaluate its suitability I did the following:

• Generated a 5mVpp 1.3kHz signal with an SR785 and fed that into CM board In1, all boosts off, 0dB AO gain. 
• Both BNC and LEMO connected to CM servo out
• One of BNC or LEMO connected to IN2 of MC servo, input gain of 30dB but disabled, OUT2 switched to AO and fed to Agilent spectrum analyzer. 
• Terminated MC IN2 for comparison. 

No real difference was seen between the two cases. The signal peak was the same height, width. 60Hz and harmonics were of the same amplitude. Here are the spectra out to 200k, they are very similar. 

Mode cleaner was locked during this whole thing. This may interfere with the measurement, but is similar to the use case for the AO path. If ground loop / spurious noise issues keep occurring, it will be worthwhile to examine the noise of the CM and MC servo paths, inputs and outputs more carefully.