Don't get me wrong, I think this would be a great thing to have and I'd love to get involved. For audio playback there is Mopidy (I'm a maintainer) which provides something similar but it would be so much better if there was a open chromecast-like protocol we could hook into and integrate into 1st-party apps rather than requiring the use of our clients.
I have intentionally made this the same as things work with ChromeCast.
I understood it that I can't use the actual soundcloud app with all its functionality that I'm used to. The same app I use when I'm playing music on my phone. I have to use your client app as that's the one which includes support for nymphcast. That's your client app where you've re-implemented some subset of the functionality I want. Soundcloud could integrate the nymphcast client sdk into their app and then I'd have the same experience I get now with chromecast, but why would they do that?
As they say, you already got the 'no', so why not try to get the 'yes' or 'maybe'? :)
For what my failed experience is worth, try getting some internal contacts at these companies. The public, and even "developer", contact channels for Spotify, Soundcloud, Tidal and TuneIn are a dead-end. At least, that has been my frustrating experience. Presumably if you throw them money they take you more seriously.
On the client side having it integrated into the 1st-party client apps would be great, yes. Right now in the SoundCloud app I implemented a basic track/album/author search function that uses the server-side app to search through the SoundCloud database.
Though barebones right now, it provides essentially the same functionality as using the SoundCloud website directly. I can imagine this approach working in a generic fashion, assuming some network-accessible API is available that provides this information.