Hi Dave,  unfortunately, that cable is designed so that the RJ45 (square connector) end goes into their Tigertronics  Signal Link output, with the round DIN connector going to the rig’s data/packet port, not into the rig’s mic input.  Its unintended use could damage the rig.  

Your Yaesu SCU17 adaptor is great for CAT and digital tasks, but not voice.  Perhaps one workaround would be to use its backpanel audio in/out connection, but for mic interfacing, it would require a custom cable with a matching circuit - minimally a resistor and a capacitor in it.  Instructions from Yaesu are limited for its audio in/out connector, Perhaps there is someone here smarter than me (just about anyone else) who could recommend a way to match its voltage/impedance for creating a cable split from the SCU17 audio in/out port, with one split going to RigPi RX In (receive) and the other to the rig RJ45 mic connector .  

  As an alternative to that, I have posted a 1 pager “how-to” in the forum Yaesu folder for a direct RigPi TX Out to Yaesu FT450D mic input cable kindly provided me by N7OZ.  That would be an alternative to trying to pull audio from the SCU17.  I imagine that it would work fine for your application as well.

74, Steve KE4LC