On last answer...
> perhaps more importantly they could be setup with 2 of these in a
> machine as there's plenty of space so if one failed, the firewall would
> failover onto the second device.
m0n0wall presently uses the master device on the primary controller.
The extra flexibility could be developed, but it requires
expertise in FreeBSD, as the tools to do it are (intentionally)
not included on the tiny image that is used for real firewalls.
If you have the expertise, there is information on how to
work on this on the web site.
(PS I'm just a satisfied user, not one of the developers.)