So just to report back for posterity. I've tried the latest 5.18 kernel for the 'rolling' build on https://www.armbian.com/rock64/. No luck with the DAC HAT, audio out the 3.5mm jack no problem. Back to the legacy kernel for me...
The list of devices looks promising, but none of them produced sound out the i2s HAT.
# aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: Analog [Analog], device 0: ff010000.i2s-rk3328-hifi ff410000.codec-0 [ff010000.i2s-rk3328-hifi ff410000.codec-0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: HDMI [HDMI], device 0: ff000000.i2s-i2s-hifi i2s-hifi-0 [ff000000.i2s-i2s-hifi i2s-hifi-0]
Subdevices: 1/1
Subdevice #0: subdevice #0
# aplay -L
null
Discard all samples (playback) or generate zero samples (capture)
hw:CARD=Analog,DEV=0
Analog, ff010000.i2s-rk3328-hifi ff410000.codec-0
Direct hardware device without any conversions
plughw:CARD=Analog,DEV=0
Analog, ff010000.i2s-rk3328-hifi ff410000.codec-0
Hardware device with all software conversions
default:CARD=Analog
Analog, ff010000.i2s-rk3328-hifi ff410000.codec-0
Default Audio Device
sysdefault:CARD=Analog
Analog, ff010000.i2s-rk3328-hifi ff410000.codec-0
Default Audio Device
dmix:CARD=Analog,DEV=0
Analog, ff010000.i2s-rk3328-hifi ff410000.codec-0
Direct sample mixing device
hw:CARD=HDMI,DEV=0
HDMI, ff000000.i2s-i2s-hifi i2s-hifi-0
Direct hardware device without any conversions
plughw:CARD=HDMI,DEV=0
HDMI, ff000000.i2s-i2s-hifi i2s-hifi-0
Hardware device with all software conversions
default:CARD=HDMI
HDMI, ff000000.i2s-i2s-hifi i2s-hifi-0
Default Audio Device
sysdefault:CARD=HDMI
HDMI, ff000000.i2s-i2s-hifi i2s-hifi-0
Default Audio Device
dmix:CARD=HDMI,DEV=0
HDMI, ff000000.i2s-i2s-hifi i2s-hifi-0
Direct sample mixing device