I don't know what Stu needed the connection from the side of that crystal for.
No schematic of Stu's device here, so you'll have to ask him what that was all about.

But here is the layout of the I2S (aka. IIS) header inside the Mk2a, along with the nearby 3V power source.

Stu appears to be using IISC, IISW, and IISD1 from the header.
Ground is also available there, so no need to go hunting elsewhere for it.
Those are the same pins we'll want to connect to header J12 on the BT dev board (SCK, WS, SDIN, GND).