I can guarantee you with absolute certainty, that the HARDWARE is definitely capable.
The issue is that the software has not been written for it.
It will require some changes in the kernel, both to the pin configurations, as well as to the audio driver.
Not only that, but you could actually do two stereo pairs of inputs, outputs, or one of each.
On the low speed header;
27: Data1. Note that this pin has been repurposed as "GPIO-E", but it is still connected.
If you refer to the register documentation around page 3058/3059, it refers to the direction that the i2s lines are configured for. 0: SPK direction -- output, 1: MIC direction -- input, 2: BIDIRECTIONAL!!!!