I have looked into this, using the latest images for Openelec and OSMC and the only step to take (besides choosing the right output device) is to add the following to /boot/config.txt:
dtoverlay=hifiberry-dac
as to how to add that line to /boot/config.txt, you may either edit the file on the SD using another computer, or ssh into it… both methods worked for me, OSMC in particular did not complain.
Incidentally, both OSMC and OpenElec support the Raspberry Pi Official Touchscreen out of the box (including lcd_rotate=2 flag)