More or less you are looking for a gateway to the public side of AllStarLink. Any type of node would work on the public node. i.e. hub or radio interface. The private node could connect to your public node. Then the private node would have to pass commands to your public node to get pass it. i.e node 1001 wants to connect to 2000. it would connect to your public node first then pass commands to connect to 2000.
Too answer your question about multiple node numbers on a single interface. No, I don't remember more then one node number per interface.
Sounds like you are looking for a simple way to register your private nodes for your network. Making your own registration server wouldn't be too hard. Re-configuring your private nodes would be more work then making one file and coping to each private node. I can't remember if #includeifexists statement works with the rpt.conf file. But you could add a small script to download a private node list and restart asterisk.
You could also make them all public nodes and try blacklist and whitelist.
David
KE6UPI