What are options for an optically-isolated power supply?
I'm guessing that you'll be lucky to get 1% end-to-end efficiency with reasonably priced parts- maybe more like 0.1%. if that doesn't bother you maybe try 1W (or 3W) white LED or LEDs coupled to a fairly decent PV panel and some electronics to regulate the output to 3V. 

There was a COTS fiber solution to this (a few hundred mW), but if I recall correctly price was several thousand dollars. IR lasers can have higher efficiency than LEDs, and they can be tuned to the PV wavelength. 

Transformers are definitely going to be more efficient if you can find a way to use one. Unless you use light pipes or fibers to couple the light the large surface area of the emitter and PV cell may result in a significant coupling anyhow.