How to open up Airplay to the gateway machine?

I have a wireless connection to the internet, and am using my MBPro to get online. The wireless ISP uses MAC address verification; I am only permitted to connect one device directly to the wireless network, due to "wireless overhead issues." Or some such. I am permitted, however, to create my own network, provided I only connect to their network using one connection.

So here's what I've done: I also have an iPhone, an iPad, and an AppleTV, all connected to an Airport Express base station, CAT-5'd to my MBPro as its gateway to connect to the internet through the MBPro's wireless connection. I am using WaterRoof for NAT.

Here's the thing. I can stream from my iPhone to the AppleTV, and from the iPad to the AppleTV, no problem. Airplay works like a champ. If I disconnect my MBPro from the internet, and connect to the Airport Express's WiFi, then I can stream from my MBPro as well. But when my MBPro is connected to the internet, and acting as a gateway, it can't find my AppleTV.

I'm assuming this is a broadcast issue or something, and that I need to tell my Airport Express to allow this communication from the gateway into its wireless network. Anyone know how I would do that?

Thanks in advance,
DS