Leon,
While I think the propsal from Nick might be the easiest to implement as a domoticz triggered script on the RPI, I just want to say (Slightly off-topic maybe): Do you know that you can run Domoticz on ClearOS too, either as a replacement for the RPI (if you are not using RPI specific stuff etc) or as a complement running both of them in a master-slave config? I am maintaining a package in the clearos-contribs repo. There is also an app in the clearos market place if you prefer an easy setup.

I have however not tried to run master-slave configurations, so I can not give you any advice on it.

/Fred

ssh root@clearos_IP 'shutdown -h -t 5 +0 & exit'

Seemed to work pretty instantaneously and closed my ssh session gracefully.

I don't see why you can't. Set up ssh keys - private on the Pi and public on ClearOS - then from the Pi send the command:

ssh root@clearos_IP 'poweroff'

It is a bit brutal as if forcibly closes the ssh session rather than politely. You could instead try something like:

ssh root@clearos_IP 'shutdown -h +1 & exit'

This gives a 1 minute delay from issuing the command. I am not sure how to get the delay smaller as the increment is in minutes. +0 is equivalent to the "poweroff" command.