I have tried it when using the J1(40PIN Header) which did not worked, I have no clue about using the J3(FPC Connectors)
I’m sure you can do directly from gpio of the raspberry. clearly the heading changes according to the distribution used. I use volume and I asked on the forum of volume.
on the gpio there are necessarily the i2s pins. you can try to make the right connections with i2s of khadas tone board and set i2s dac generic. if there is anything to change in the file I’m not good at that.