I was thinking that the "FOSC" product line (which is called a "coupler" instead of a "splitter/combiner") was what we wanted. 

Koji brought to my attention that the 90/10 splitters we already have are of this line. So, I rigged a few up to shine a hopefully beating pair of fields on the fiber coupled thorlabs PD. 

I was able to get ~80uW each of PSL and AUX X light on the PD, which produced a -10dBm beatnote!  Thus, I think these FOSC splitters are indeed what we want. 

I then threw this IR beatnote at our ALS signal chain. The beatnote was too big to throw through our ~+27dB RF amps, so I just sent the -10dBm over to the LSC rack.

The IR beat spectrum is somwhat noisier from 10-100Hz, but, more interesting, is that the sub-4Hz noise is identical in the two beats, and very coherent. This excludes ALS noise arising from anything happening in the green beat optics on the PSL table.

Obviously, the high frequency noise is largely the same and coherent too, but also coherent with the AUX X PDH control signal, so it is understood.