I2S in slave mode

[unig47]

Hi all
I am currently developing a new hardware where the output of a Web radio shall be sent to an (Analog Devices) ADAU1701 Signal Processor for further processing. At this point many thanks to @schreibfaul1 for his excellent work! The ADAU1701 sports several I2S inputs but their clocks (BCLK and LRCLK) must by synchronous to its internal master clock, which is a crystal oscillator. Now this is NOT synchronous to the master clock of the ESP32. If the ESP32 could be set to slave mode the ADAU1701 could supply the BCLK and LRCLK and all should be fine, I thought. So I tried to modify the constructor of the Audio class in this way:
// m_i2s_config.mode = (i2s_mode_t)(I2S_MODE_MASTER | I2S_MODE_TX); // original code m_i2s_config.mode = (i2s_mode_t)(I2S_MODE_SLAVE | I2S_MODE_TX); // modified code
Obviously the solution is not that simple, because now now there is no output data at all although both BCLK and LRCLK (48 kHz) are there. Ok, BCLK does not have 16 bits per channel, but 32. ADAU1701 can only supply this. As far as i understood the Philips I2S format, this should not be a problem - or is it?
Any help is greatly appreciated.

[unig47]

In the meantime I found in https://docs.espressif.com/projects/arduino-esp32/en/latest/api/i2s.html that

In Slave mode the device listens on attached pins for the clock signal and word select - i.e. unless externally driven the pins will remain LOW. This mode is not supported yet.

Hm, there we are. Does anyone know if and when I2S slave mode might get supported in the Arduino universe?