I'm not very familiar with OpenElec but my understanding is that it is quite a restrictive environment out of the box, yes.
I found the following which should hopefully be of use to you, given the underlying hardware in the pHAT DAC is identical (AFAIK):
https://www.hifiberry.com/guides/configuring-the-sound-card-in-openelec-with-device-tree-overlays/
I suspect that once you have mounted a rw filesystem you might be able to just recycle the entire phatdac script, but if not it shouldn't be too complicated either.