SBT Multi-Project Build with dynamic external projects?

Let's say we have an SBT project bar with a dependency on some artifact foo:

val bar = Project('bar', file('.')).settings(    
  libraryDependencies += "com.foo" % "foo" % "1.0.0"
)

However, in certain cases, I want to checkout the source of foo and have SBT load the source from my file system instead of the published artifact; that way, I could make local changes to foo and immediately test them with bar without having to publish anything.

val foo = Project('foo', file('foo'))

val bar = Project('bar', file('.')).dependsOn(foo)

We have a spec.json file in the root folder of bar that already specifies if foo should be used from source or as an artifact. Is there any way to setup my build to read this file and add dependsOn or libraryDependencies based on the value in spec.json? '

It's easy enough to do this for libraryDependencies:

val bar = Project('bar', file('.')).settings(    
  libraryDependencies ++= 
    if (containsFoo(baseDirectory.value / "spec.json")) {
      Seq()
    } else {
      Seq("com.foo" % "foo" % "1.0.0")
    }
)

However, we can't find any way to set do anything "dynamic" in dependsOn, such as reading the baseDirectory SettingKey.