Hi Allen,
If each device in the system has it's own MCLK (master clock-in) source, there could be an issue with passing audio from the BT module through the external DAC if each of these are master I2S devices. This can result in occasional repeated or dropped samples. The ADAU1761 does not have an integrated ASRC. Is sample rate conversion available on the BT module?
If it is possible to share MCLK with the Bluetooth, then you can set up the ADAU1761 I2S input and output ports as master I2S devices, and make the external DAC a slave I2S (even it it has it's own clock source, you might still be OK).
-JT