In SBT, how to reference a subproject of another project?

In SBT it is possible to depend directly on another project. This has the advantage that when running ~compile, a change to the dependency causes a rebuild.

Now I want to depend on a subproject of another project, without depending on any of its siblings. So, for example, I have:

a/
    build.sbt
    b/
        build.sbt
    c/
        build.sbt

d/
    build.sbt

and I want d to depend on b but not on c.

I tried, in d/build.sbt,

lazy val d = ProjectRef(file("../a"), "b")

lazy val root = project.dependsOn(d)

but this gives

Note: Unresolved dependencies path:
    com.foo:a_2.10:1.0
      +- root:root_2.10:1.0

I could of course do

lazy val d = RootProject(file("../a/b"))

except that it is possible for a/build.sbt to contain additional settings for a/b that won't be pickup up this way. I need a reference that will pick up a/build.sbt but also refer specifically to a/b.

Is there any way to make this sort of reference?