My m0n0wall device has 4 ports- 1 for the WAN link and 3 of which are setup
as 3 separate subnets (using NAT).

We have a .224 network with our T-1 service. I'd like to route traffic for 1
specific public IPs to one of the subnets, but can't see how this is done.
Is it possible? 

In Firewall: NAT: Edit 1:1 I can map multiple public IPs to multiple NAT
addresses, but it doesn't allow me to map only 1 public IP to a whole
subnet.