You could get list of interfaces via NXSL script - the script should filter out loopback interface and if there are several interfaces - select one of them. There is currently no hook script on tunnel connection, you can run your script from configuration poll hook.

Here's an example script that prints list of interfaces:

for(i : $node->interfaces)
{
  println i->name;
  for(a : i->ipAddressList)
    println "   " . a->address . "/" . a->mask;
}

Also, there's a system event SYS_TUNNEL_OPEN - this event occurs when the tunnel is established, in the context of that node. So you can create a rule in EPP and run some script from there.

It's not possible to update node's IP address from script, but you can create a cusome attribute on node and keep IP address there.