The sbt documentation always indicates that file path must be absolute, but all examples are relative to project base. That leaves it a little unclear as to how files outside of the project directory might be referenced.
I created a polyglot project with an Akka backend and React.js frontend. These were placed in separate directories since the linkage is small and at arms-length. There was no need to determine how to integrate webpack into sbt.
Deployment into the test environment required a fat-jar, generated by sbt-assembly. The fat-jar needed to include the js and html files from the webpack build process.
The solution turns out to to be simple, even obvious.
unmanagedResourceDirectories in Compile += file("../../react/build")
or
unmanagedResourceDirectories in Compile += file("/home/rsearle/work/react/build")
The assembly plugin unfortunately does not (AFAIK) allow the specification of resources that are only included during the actual packaging process.