Based on the pinout in this article (credit to Nick, M0NFE) the audio cable for Digirig will look like this:
RJ45 pin 3 (GND) → 3.5mm TRRS sleeve (GND)
RJ45 pin 4 (PTT) → 3.5mm TRRS ring 2 (PTT)
RJ45 pin 5 (MIC-) → RJ45 pin 3 (GND)
RJ45 pin 6 (MIC+) → 3.5mm TRRS ring 1 (RIG_AFIN)
RJ45 pin 7 (RX) → 3.5mm TRRS ring 1 (RIG_AFOUT)
Looks like Chirp programming is possible with logic levels configuration of the serial port
RJ45 pin 3 (GND) → 3.5mm TRRS sleeve (GND)
RJ45 pin 4 (RXD) → 3.5mm TRRS ring 1 (RIG_TXD)
RJ45 pin 7 (RXD) → 3.5mm TRRS tip (RIG_RXD)
Please report here on your build results.