You’d have to somehow gain access to the audio related CPU registers to control the I²S peripheral. See Peripheral specification from BCM2835 - Raspberry Pi Documentation
Pisound specific kernel sources are here:
- linux/pisound.c at rpi-5.10.y · raspberrypi/linux · GitHub
- linux/pisound-overlay.dts at rpi-5.10.y · raspberrypi/linux · GitHub
I don’t think there’s much to gain by bypassing ALSA APIs though. 