Have you considered spinning off the visualizer as a separate project? I'd love to it running on its own, on top of any audio I may have playing on the background

Yes, it's already its own project and the developer who built it helped integrate it into Webamp.

https://github.com/jberg/butterchurn

If you want to visualize audio coming from the browser (Spotify etc) check out his browser extension: https://chrome.google.com/webstore/detail/butterchurn-music-...