Setting output folder for unmanaged sources

I am working on a Java Play app, using Play 2.3.8 which depends on some external classes, in a project with non-standard directory structure. I want the sources to be loaded into Eclipse. Therefore I have the following in my build.sbt

lazy val beetle2Path = file(sys.env.get("BEETLE2PATH").getOrElse("/group/project/onrbee/beetle2"))
unmanagedSourceDirectories in Test += beetle2Path / "src/util"

This generates the following lines in my .classpath

 <classpathentry output="-afs-inf.ed.ac.uk-group-project-onrbee-myrosia-beetle2-src-util" kind="src" path="-afs-inf.ed.ac.uk-group-project-onrbee-myrosia-beetle2-src-util"/>

This results in Eclipse putting compiled classes together with source classes, which is undesirable, of course. Is there a way to set an output directory for unmanaged sources, so that the output could go to the "build" directory in that project tree, ideally? Or even to the default eclipse output directory?
Or am I doing something wrong with the build definition there?

Read the build.sbt settings and the generated .classpath entry for the unmanagedSourceDirectories setting. Trace how the source path and output path are selected, then verify that generated Eclipse metadata keeps unmanaged sources separate from compiled classes and uses the requested build or default output directory.