Rhapsody isn't really a player, like Kontakt anyway. It's more of a manager and launcher for instrument expansions built with HISE.
There is no documentation because a Rhapsody instrument is just a HISE instrument. However there is a bunch of dedicated scripts and a template for building an expansion that looks/feels like it belongs with other Rhapsody instruments.
I made a little intro video which will hopefully give you some idea of the process:
https://youtu.be/x4b8WlStG40
I haven't been following this thread. Can someone quickly fill me in on the details? If the samples are already prepared, perhaps it's something I could quickly put together as a Rhapsody instrument.