Play an audio file only when server is full?

[someToast]

When my server's full, I want people connecting to hear a brief, recorded audio message, but I don't want that same message played at the start of each and every stream when the server isn't full.

Could a .pls file do this? Specify the stream address first and a file in the content directory second? (And what would be the syntax for that?)

Assuming this can be done, I've never used a .pls file on the server before. If I explicitly name the file "listen.pls", will that file then be what's called up when someone connects from their existing Winamp/iTunes favorite stored in their media player (from before any .pls file existed)?