Also it could mean already legal trouble for me. And spotifies auth mechanism anf recent changes there put me off so far

I'm surprised that https://everynoise.com still works, but it does. At one point, I had heard that it was shut down.

I wonder if it could be done with torrents ... or user-supplied API key. Still some legal attack surface area, but perhaps no worse than yt-dlp.

If users type in their (premium) credentials it is legal.

But I still cannot just take someone else project and put my things on top if it ain't open source and it isn't and there seems no indication of wanting collaboration. So I could contact him if I ever made anything, but I cannot just make and release it.

Ah, I was imagining a Chrome extension would be the easy way