I think I think this is pretty cool, but I'm not sure if the reasons I think it's cool are valid. For example, I can imagine a jukebox app for parties or bars accessible via SMS, but I can't tell if Spotifyd actually enables such an app. Furthermore, if Spotifyd does enable such an app, would that app be impossible without Spotifyd?
I glanced at the readme, but it looks like it's written for someone who already knows why they wanted an open source Spotify client.
Seems like sp[0] (from the README) could be a pretty decent basis for what you want.
That said, AFAIK it's not technically legal to use regular Spotify in a public space. Spotify Business[1] seems to use a separate client, no idea whether that's instead of or in addition to the consumer ones.
I glanced at the readme, but it looks like it's written for someone who already knows why they wanted an open source Spotify client.
Regardless of all the above, good work!