Hi Boris,
This is great! You could be the first third party library to depend on Rapture JSON (that I know of)!
First of all, you will need to depend on just "rapture-json". That's the common dependency of all the backend projects. Then your users will need to depend on "siren-scala" plus "rapture-json-[backend]". There are other ways of doing it, but this saves you publishing once for each backend.
In code, I think your users would need to import the appropriate `rapture.json.jsonBackends.<backend>._` package before starting to work with Siren.
I've had a brief look at siren-scala. Do you think you would be providing your own extractors and serializers for a `Siren` type, or something where Rapture is more transparent? I'm happy to help with working out the correct extension points!
Looking forward to finding out more!
Jon