On modern JVM we can use direct NIO buffers and let Java access native memory directly at full speed. There is (almost) no overhead at all anymore, so if we do it that way, it's pretty fast actually.

I recommend Jace for that: https://bitbucket.org/cowwoc/jace/wiki/Home

No, that's because in the specific case of LLVM only the C API is currently mapped. We'd have to work a little bit harder for the C++ API, but there is interest in doing so, if only to use it as a better parser for JavaCPP itself: Improve Parser: Use the Clang API https://github.com/bytedeco/javacpp/issues/51