As stated in the first post, I've tested it on 4.9 and 4.4, resulting in same behaviour on both. I've fully disabled mcasp in my .dtsi:
but it didn't work. This may be due to the board being BBGW not BBB/BBBW - I've already encountered few other things that work differently and are poorly documented.
Also, the aim was to do it via pure DTB without any plugin/overlay (dtbo).
As I've written already, I've just switched to SPI0.
Additionally, the 4.9.x kernel seems to have some other ongoing issues (e.g. PRU RPROC not working in 4.9.32, bad wireless adapter MAC address in slightly earlier versions and so on) so I've reverted to the most up-to-date 4.4.x.