HDMI is supported with Maemo Leste.

With N900 you need composite video to HDMI converter. With right cable you can use Scart to HDMI converter.

It is possible use USB-C Docking Bar with HDMI port with Pinephone.

Hint for pixel perfect mirroring Pinephone screen to HDMI monitor that has little different screen size use command:
xrandr --fb 1440x768 --output HDMI-1 --mode 1366x768 --panning 1440x0
After command you missing drawing bottom 48 pixels in Pinephone screen and with panning you can see right drawing 44 pixels in monitor.
And going back to normal:
xrandr --fb 1440x720
Suprise was that after that my monitor continue show screen resolution in 1366x720 with black border in bottom although xrandr says that is not supported.

I noticed with Pinephone Keyboard that the best order use HDMI is:
1. Start Pinephone Keyboard.
2. Connect HDMI cable to Docking Bar.
3. Push two times charge button (right side of keyboard) to stop use secondary battery
4. Connect Docking Bar to Pinephone side USB port.
5. Use xrandr command

If xrandr say resolution not supported check that TV or monitor does not sleep.