To attempt to shed light on my own question. Custom post processing on the Quest in Unity doesn’t seem to be possible, and there is no word on if/when it will be possible
long open issue here :
https://github.com/Unity-Technologies/PostProcessing/issues/717
The official URP docs confirming that post processing v2 doesn’t work. No details of a replacement :
https://docs.unity3d.com/Packages/com.unity.render-pipelines.universal@7.1/manual/integration-with-post-processing.html
According to this post there is a way to do post processing in the HDRP, so there is hope this will be ported to URP, but I can’t find anywhere that says it will :