That should work fine. You'd just have to make sure the connection you were trying to share can handle that set-up. Some broadband connections have to be connected directly to the router (in this case, the Airport BS) in order to function.

In my case, I use a Linksys Router to do the PPPoE connection to my DSL server (handling the assignment of IP addresses) and I have the ABS connected to it providing a wireless connection to the laptops, but no IP or NAT services.