Actually I was doing that, I only made a mistake simplifying the problem for demonstration purposes. On the master branch I have set up the whole SBT tool chain using the sbt-jni plugin, which comes with a javah interface for Scala. In my original version I obtained the correct signature from that. By the way, I'm actually pretty happy how well Nim integrates with the Scala+SBT world.
I just verified: Using the correct for the simplified example does not make a difference.
