Could we get this working with the duet?
It looks like duet might already give the required action command.
https://duet3d.dozuki.com/Wiki/Gcode#Section_Replies_from_the_RepRap_machine_to_the_host_computer
I've posted a similar request on the duet forums for getting this working
That's going to have to be done by the Duet firmware folks. We have no control over what they do.
BTW, there is a plugin that will allow you to handle custom action commands if the Duet firmware doesn't use already supported commands...
If it's mentioned in the documentation, it might be an optional feature that has to be enabled. Not sure how that works for the Duet/RepRapFW
the action commands menu is not available to me in the settings to be able to assign custom actions 
It might ne python 2 only, check their github repo and see if there is an issue, and potentially other forks that have been made python 3 compatible.
I was confused and didn't realize that was not the pre-installed plugin but an additional one. It seems that duet at the moment only supports 3 commands.
" On the latest version of Pronterface and Octoprint (1.2.0+) a special comment of the form:
// action:command
is allowed to be sent from the firmware, the command can currently be pause, resume or disconnect which will execute those commands on the host."
Those are handled by the default bundled Action Commands plugin for sure. So it would just be a matter of configuring your runout to send the pause action. Not sure how to do that in RepRap Firmware but hopefully that gives you something to search for how to do.