An optoisolator requires a current flowing through a photodiode to turn on a phototransistor so that the latter will conduct.

Neither side has to be connected either to power or ground, does it? They'll work fine if they are, but there's no requirement, so long as the requisite current flows through the input circuit. Once that happens, what you do with the output circuit is entirely up to you. I've used devices with greater than 2.5 kV of isolation, which produce a nice current loop relative to earth-GND potential while controlled at a 2kV potential from yet another current loop.

That's what ISOLATION means.

RE