I also did a design using a 74HC393 to generate the I2S clocks, then an SPI module to create the data stream:
12.288MHz crystal on the microcontroller, tapped off to feed the CK on one half of the 74HC393 (the two halves were cascaded).
Then tapped off the 3.072MHz (BCK) and 48KHz (LRCK) for the converter. The BCK and LRCK also fed to the SPI module.
Depending on the DAC, the MCK can be 12.288MHz (256fs) or 6.144MHz (128fs), be careful tapping the crystal oscillator (that you don't kill it) - I've done it in commercial designs a few times (running a dsPIC and wolfson DAC off the same crystal) with success, but you might want to buffer things.