(In reply to comment #0)

> #2, without xorg.conf
> The receiver sends its own EDID block, so I can extend via xrandr without
> turning on the projector.
> But the projector is turned off in most cases, so is there a way to disable
> a display output but keep the HDMI audio output? (as a workaround for #1)

Nope, unfortunately not. It's by HDMI design, the audio signal is mixed into
the vertical and horizontal blank periods of the video signal -> so if you
don't have a video signal you don't have audio also.

As a workaround you could try to setup a second X server or framebuffer or
something like that (kms console?) on the HDMI output to just get a video
signal on it.