I can't follow your description - you obviously know what you are talking about, but I missed it. I am taking it to be:
You have box A with a standard internet feed, and another nc. Box B you want to connect to box A, but not the internet.
The way I would do that is to put Box B in /etc/hosts, add a static route to it, and then connect to the internet, leaving the default route as the internet. You can control whether Box B has internet access or not by enabling/disabling ip forwarding with a command.
Code:
echo 1 > proc/sys/net/ipv4/ip_forwarding
echo 0 to disable, 1 to enable. Maybe do it with ip[v6 as well if you are futuristic :-P.
Then I fix whatever didn't work :-)).