The LOC110 has two optical detectors, the first for normal output, the second to use as feedback on the input side to adjust the input current. If you don't use this particular mechanism then you could use any optocoupler. The problem is knowing the characteristics and the large variance across devices so each complete unit would have to be individually calibrated. (I am told switchmode power supplies can use regular optos in an analog mode as part of a feedback loop.)