Getting the Winamp song isn't that hard, there's an example posted on the NSDN (
here, at the bottom, just have to convert it to Delphi). If I'm correct (but don't quote me on this though) you can return commands from your plugin which will be processed by Plus!. I have not yet tried this, so I can't be 100% sure. The problem with nickname changers is that plugins react only on commands typed by the user, so for the song-nickname to work they'd have to type in "/xyourcommand" every time they want to update it... this would make your plugin quite obsolete, as they can just use "/nick (!WAT)"...
I ran into the same problem when people started asking me about nickname-changing for WMPlus. Since I didn't want that option anyways I out-sourced it to a friend, which will create a standalone application to do it. The other option might be to create a new thread in Initialize and have it check for song changes every once in a while. I haven't verified if this does indeed work, but in theory it should...