Finally found some more useful info: seems it’s currently not possible with this prime setup where the nvidia output goes to the intel chip and then to the display.
$ xrandr --listproviders Providers: number : 2 Provider 0: id: 0x1b8 cap: 0x1, Source Output crtcs: 0 outputs: 0 associated providers: 1 name:NVIDIA-0 Provider 1: id: 0x1df cap: 0x6, Sink Output, Source Offload crtcs: 3 outputs: 4 associated providers: 1 name:modesetting
Useful links:
https://bbs.archlinux.org/viewtopic.php?id=251714