So I managed to enable external HDMI on #pinephone , fidling with wlr-randr.
I'm writting this on firefox in pinephone with a big screen and a keyboard :)
The magic spell seems to be: with wlr-randr turn off internal screen, turn off external htmi, turn back on external htmi. This way I got a signal on my external monitor.
(I ran that commands on a ssh session from the laptop)
The phone screen remains off. If I turn on the internal screen, I lose the hdmi.
When I unplug hdmi, internal screen turns on but wrong, and phosh is missing. I have to restart phosh from ssh (sudo systemctl restart phosh).
I have managed to have both displays turned on while trying to get phosh back after a crash with HDMI on. So probably there is a more powerful spell which is a level too high for me...