This is a reasonable request but unfortunately the rocket-chip codebase is not well factored into discrete Scala project build units. We have already done some work to factor out the Config library into its own Scala project (and github repo), as well as recently switching to depending on Chisel and FIRRTL as published .jar dependencies, rather than source code dependencies. I think you will see more work along these lines in the next couple of months, which could indeed include a separate Scala project for diplomacy. But as of this moment the answer to your question is No.
I recommend making specific feature requests like this as opened Issues in the rocket-chip github project, as this mailing list contains many developers of many different RISC-V implementations.