Comment 6 for bug 794503

Ahah, I was missing the dmesg line:
[ 127.878392] vga_switcheroo: client 0 refused switch

Stopping X and performing the switch does allow switching to the nvidia GPU ("DIS"crete):
# cat /sys/kernel/debug/vgaswitcheroo/switch
0:IGD: :Pwr:0000:00:02.0
1:DIS:+:Pwr:0000:01:00.0

and picking up the screen:
$ xrandr
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
VGA-2 connected (normal left inverted right x axis y axis)
   1024x768 60.0
   800x600 60.3 56.2
   848x480 60.0
   640x480 59.9
HDMI-2 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 531mm x 299mm
   1920x1080 60.0*+ 50.0
[...]

So it's now almost there, I can use Intel xor nVidia GPUs but not both at the same time.

I don't know what the VGA plug that xrandr is picking up is, I can enable it but it doesn't show up anywhere and I have no external VGA plugs.