After much struggling and researching, I've been unable to figure out how to use I2S as a bidirectional multi-slave bus for audio. Maybe it can be done, and maybe it can't; I'm just not able to figure it out.
I've thus decided to set this project aside and turn my attention to an Arduino Shield that combines this with the EchoWing. That will bypass the need for I2S, taking the processor out of the audio flow altogether and leaving it to control the effects and other functions.
I'll supply a link in a separate log here once I've started that project here on Hackaday.