Well I got it working in vanilla raspbian/pixel. Very easy once I found out how!
I haven’t tried this in OSMC so it’s a little bit OT here but since the thread exists I thought I’d add the info in case it is useful to anyone.
I used rpi-update. description and info on that is at: GitHub - Hexxeh/rpi-update: An easier way to update the firmware of your Raspberry Pi
this gives you the latest bleeding edge kernel/firmware without having to change repos
this updated the kernel to 4.9.23 and the boss dac overlay then appeared in the /boot/overlays directory
then I had to add a line to config.txt in the /boot/ directory
allo-boss-dac-pcm512x-audio.dtbo
I had thought/hoped that this line would have been added but commented out by rpi-update but it wasn’t and had to be added manually. no problem though.
reboot after all that and it seems to work fine. Of course it may have broken something I haven’t noticed yet.
I’d be interested to know if this procedure would work on OSMC or if the kernel and boot setup is too different from vanilla raspbian