Hi Neil,
Yes, that is one way that NAT can be performed. And you are correct that the Business Hub can't be configured to do NAT like that.
If you use the Business Hub to set up the machines on the public IP addresses then then 81.x addresses will be accessible from the LAN as well as the Internet. It will still be doing NAT for any other machines on the LAN to be able to access the Internet.
Thinking about it though, unless you have a decent switch, LAN<->LAN traffic will always be bouncing off of the router regardless of what IP address the server has.
Adrian