Shorewall masquerade on dynamic interface

I have a list of interfaces to which I want to masquerade traffic to. The problem is only one of them is active at once. They all match the regex vlan14[0-9][0-9]. How can I get shorewall to masquerade traffic destined out any of these interfaces based on which is active at the time?

More specifically they are all nearly identical networks (they're access ports for a product). They are all the same subnet, and use the same IPs, but they're different machines. That's why they're their own networks, and why only one can be active at a time.