I have a library compiled to a jar (not an sbt project, just the jar file) that's not available on a repository.
Is there a simple way to add a reference to the jar in the filesystem/project directly?
You can put the jar in your project's lib folder (create it if it doesn't exist), it will then appear on your project's unmanaged-classpath.
To publish a jar file locally, if you have an sbt project that produces the jar, it should be as simple as invoking "publish-local" to publish the jar to your local ivy repository so that you can use that jar in another one of your projects (on the same computer).