The way to do this is have one Bitcoin client for connecting to the public P2P network, where the external Bitcoin port is port forwarded to that NAT IP address. Then set all other Bitcoins to only connect to that machine's internal IP address using the -connect command line or config option.
Source:
Right now there isn't a port number setting to do that. It's a feature yet to be implemented. You can only set up your NAT to port-forward to one of the computers. (I said something earlier about NAT port translation, but that wouldn't work, other nodes wouldn't know to connect to that port)
If you want, as a small optimization, you could run the rest of your computers as:
bitcoin -connect=<the IP of the first computer>
so they get all their network communication from the first computer and don't all connect over the net individually for the same information. This saves bandwidth, although it doesn't use much bandwidth to begin with, so it wouldn't really matter unless you had tons of computers.
For redundancy in case the first computer goes down, you could have two that connect out and the rest connect to both of them. The first two are run normally, the rest are run like:
bitcoin -connect=<IP1> -connect=<IP2>
