Pete:
I have seen very linear opto-isolators in a end, custom made
AD converter. Sadly I couldn't get a diagram or even pop the
cover to study the magic. A Advark like USB audio capture
device that had the audio input completly isolated for hum/noise
control. It used 6 "D" cells that where supposed to run it for at
least a year. It was very clean, 24 bits. I hoped we would buy
several. Too pricy for our budget.

I will check around and see if any one remembers the make.

Terry