I'm not sure I understand what you say about cloning one mac address at a time.
Anyway, it can be done on Linux by setting the NIC to promiscuous mode. Mac addresses can be generated with this script
http://www.easyvmx.com/software/easymac.sh.There must be a solution for this in FreeBSD.
An other solution would be to set Linux box directly on the Internet and having Linux allocating the IP-addresses. When allocated by dhclient, I will make the IP addresses static from my ISP's self-service-web.
I'm sure a solution exist, but the stock m0n0wall won't do this without a hack.
Cloning one MAC, meaning you can set "one" MAC address for the WAN Nic in m0n0wall, but you can't set 2 or 5 for example.
Another solution, check the equipment that the ISP gave you (you never mentioned what it was, like DSL, Cable, Fiber, etc.) and see if this isn't already supported by the device, then just DMZ all of them to the same WAN interface.