Be sure you're all talking about the same thing. The zefiro/denecke AD-20 has optical outs like they say. The Graham-Patten ADC-20 (that many people mistakenly call the AD-20) does not have an optical output.
No idea on how difficult it would be to add an optical output to an ADC-20 or DMic-20. Part of it is logistics -- finding a place to fit it, making a hole in the case, attaching the optical out to the case, etc. Part of it is the electrical requirements. If you can find a place on the board that has the digital signal with 5v for a digital 1 and 0v for a digital 0, it'll be much easier. But the spdif spec is for 0.5v for a digital 1, which is too low to make optical transmitters read it as a digital 1. So if G-P follows the spdif standard, you can't just take the digital signal from where it feeds the BNC digital output on the back.