Adding a coaxial output requires more buffering from what I can tell vs just adding a Toslink out.
I used this board awhile back when I modded my CLD-M90 for both types of digital outputs and it worked great.....

Unfortunately, I can't seem to find these anymore and the only one's available are either a version that also adds an XLR out or a version that takes in an I2S input that I will confirm right now would not work (and it would be overly complicated to integrate).
In respect to your idea, not sure that would work. The output from the chip on the Pioneer processor is electrical in nature so you would need to use the coaxial form of transmission which still needs some kind of buffering. I would say it's not worth doing as you are over complicating it and in reality the difference between Toslink and Coax really isn't distinguishable.
Going back to to the modder for the Pioneer, his idea works great but I would add a 0.1uf cap between the +5vdc & GND as well as a 75 ohm resistor between the digital out of the IC and the digital in on the Toslink transmitter for more protection.