OpenWRT has a driver for the Raspberry Pi Audio Injector soundcard that uses the WM8731, however, it utilizes the Raspberry Pi's Broadcom I2S and I2C driver. Starting at line 297, you can see it here.
So, the goal here should be, at least for the software side, is utilize the ralink-i2s driver (I believe it's called that) and the Omega's i2c driver in the Audio Injector driver and build it for the mt76x8.
On the hardware side is to adapt the pins to connect the spi, i2s, and i2c lines to the Omega2.
Onion team, have the i2c and i2s audio issues been fixed in the latest firmware?