I have no idea how to get the real interface info up into Java

Apparently, you can't. It's BugID 4691932. (Here's a real hacky class.)

Supposedly, the Java VM will transparently translate 255.255.255.255 into a real broadcast address, but which one it picks is beyond me. In reality, I don't see this being any different than what the OS is going to do on it's own, so that may have just been speculation based on empirical evidence.