a.k.a. Suspension Point Interferometer, this is an old idea of Ron Drever's. He has made some versions of this kind of thing over the years. Other instances are the ones built by Youichi Aso for his PhD work at the University of Tokyo and also by Kenji Numata at GSFC for use in making an ultra-stable low frequency reference for measuring laser frequency noise.
For LIGO, there are a couple different notions: for Advanced LIGO we would ~4" dia. mirrors on each of the test mass holding optical tables or alternatively bolted onto the suspension frame. The mirror would be super-polished but maybe just using a commercial blank from CVI. We would pick off a piece of the main beam, frequency shift it by at least a GHz (maybe even double it) and use this frequency shifted light to lock the seismic platforms together at frequencies below ~3 Hz. This would make us largely immune to the large low frequency motions from earthquakes.
At the 40m, we would do a very similar thing, but use instead the Stacis PZT system to actuate the seismic stack.