Firewall/NAT

I'm aware per the FAQ that m0n0wall doesn't support load balancing or failover for two WAN interfaces.

What I'm not clear on, however, is: Is it possible to set up the OPT interface as an additional WAN interface and route traffic through that gateway via outbound NAT rules?

You could NAT out an OPT interface, but would require static routes to direct specific destination networks out the OPT interface since you can't have multiple gateways, and policy routing isn't an option.

It wouldn't be very functional at all, but it would be possible to do it that way.