Hi Felix,
Unfortunately I don't understand JSON RPC 2.0, but it feels odd to me that you'd need an endOfRequestResolver for so specific a protocol. TCP is a transport protocol that can create multiple different application protocols, so it needs the endOfRequestResolver to resolve to custom application protocols (e.g. HTTP uses, for example, the Content-Length header, but a custom protocol may use the first four bytes to record the length).
That said, check out the isEndOfRequest function
here to see how it's used for the tcp protocol. Mountebank will pass all imposter header level information to the custom protocol implementation, so if the user has defined that field, you'll have access to it.
-Brandon