Question: Suppose I have a settings file:

includeBuild("myPlugin")

And a plain old script:

buildscript {
  dependencies {
    classpath("myPackage:myPlugin") // included build 'myPlugin'
  }
}

Would dependency substitution work in this case, or will Gradle barf when the script is loaded because it can't find the included build?

Not sure from the top of my head, but why would you want to do that instead of properly using the plugins block?

I actually had a similar question if the included build was not a plugin as well.

You mean that you want the included build in the build script classpath without there being a plugin? The slickest solution is to just have a plugin that does no actions and thus just brings the classes to the build script classpath.