To investigate further, I downloaded the current Raspberry Pi OS and tried that with the monitor plugged into the Pi again. It worked - but I saw nothing until a cursor arrow appeared and then X started.
So it appears that the RPiOS can find the monitor while Devuan cannot - though it may be X that does the finding rather than the core system. I have not tried X with Devuan.

Getting this far reminded me that this monitor is an old Philips Monitor that has a DVI input rather than a true HDMI input. There is a socket converter permanently plugged in so I just use a standard HDMI cable and had forgotten the subtlety.

The monitor does work normally with an RPi4 and Devuan runs X very nicely - but I set this up long enough ago not to be able to remember the process. I am sure there were not many hurdles though

I always intended to use the Pi0 headless  and was pursuing this problem more as a matter of principle than need. It now looks as though it is too niche to be worth pursuing further.  Unless anybody wants to pursue this, I consider this solved.

Thank you c0rnelius for your help.

I just spun this and tested it (worked fine for me): https://github.com/pyavitz/rpi-img-buil … -16.img.xz
First boot and general usage: https://github.com/pyavitz/rpi-img-builder#usage

I also included a default passwd for root: toor

I downloaded devuan_chimaera_4.0.0_armel_rpi0.img.zip from the arm download page, checked the sha256 value (it was fine) and loaded it onto an SD card. I modified the "change this to Credentials.txt" file appropriately and booted the pi.

The attached hdmi monitor wakes up but only displays a plain black screen but no boot messages, no init messages and no login prompt.

I can ssh into the pi perfectly well.  Looking at the log files:-
    dmesg mentions a framefbuffer and a console but shows no errors. No mention of hdmi.
    boot(.log) shows no errors but no mention of hdmi.
    kern.log has mention of vc (videocore?) but mostly in relation to video capture. It also appears to reserve video memory. No mention of hdmi.

Going back to the raspbian SD card, everything works including the monitor so the hardware is ok.

Searching the forums, I see reference to /sys/devices/platform/drm/ - and a number of directories below that one of which has hdmi in the name. There is no drm directory in the pi's /sys/devices/platform directory. But it is not there on a Pi2 that I converted earlier - but that does not have a monitor plugged in. The suggestion was that it should be possible to read the edid for the monitor from that location.

I suspect that my pi does not know it has an hdmi connector or a monitor connected to it.

I also found a post saying that some firmware should be copied to /lib/firmware/brcm to solve this problem - but try as I might, I cannot find that post again, and it was not very specific about which firmware to copy.

Any help, suggestions or pointers welcome. Preferably, I would like to understand what is wrong - not just fix it.

Thank you