TCP/IP uses broadcasts - simply send your packet to x.x.x.255, for a Class C example.
We route NetBIOS across distinct networks to enable NT4 boxes to participate in SMB browsing. The only caveat is that you must disable the Computer Browser service on the box doing the routing, or else use a dedicated router. (The reason being that on a multihomed box, if the Browser service is running, it won't propagate NetBIOS browse traffic across interfaces.)