Supporting a new toggle to send on song leave (or however you want to word it) is easier than that (although I need to add a new database column for this). Smart buttons have to be positioned somewhere on each page, and each page can be completely different sizes, so there really wouldn't be a good way to handle adding a button to every page in the entire library (unless it's just anchored to a specific size with a margin, but that still requires a new UI for configuring, and all the code to update every song in the entire library to add a button). This is also not really something that I imagine would be useful to most users, but if lots of users ask for it, I can always consider it.
Mike
Mike