Best practice for extensible SBT plugins?

javap-out.txt

harlan@plato:/tmp/scalamixin$ javap MyProject
Compiled from "mixin.scala"
public class MyProject extends BasicProject implements QUnitSupport,scala.ScalaObject{
    public scala.collection.immutable.List qunitSources();
    public void QUnitSupport$_setter_$qunitSources_$eq(scala.collection.immutable.List);
    public void qunitRun();
    public scala.collection.immutable.List jsSources();
    public void JavascriptSupport$_setter_$jsSources_$eq(scala.collection.immutable.List);
    public MyProject(ProjectInfo);
}

harlan@plato:/tmp/scalamixin$ javap MyProject2
Compiled from "mixin.scala"
public class MyProject2 extends BasicProject implements JavascriptSupport,QUnitSupport2,scala.ScalaObject{
    public scala.collection.immutable.List qunitSources();
    public void QUnitSupport2$_setter_$qunitSources_$eq(scala.collection.immutable.List);
    public void qunitRun();
    public scala.collection.immutable.List jsSources();
    public void JavascriptSupport$_setter_$jsSources_$eq(scala.collection.immutable.List);
    public MyProject2(ProjectInfo);
}

mixin.scala

// ============= Dummy Code =============

class ProjectInfo {

}

class BasicProject(info: ProjectInfo) {
	val sources = List("src/main/scala")
	val resources = List("src/main/resources")
}

trait JettySupport {
	def jettyRun =  ()
	def jettyStop = ()
}

class WebProject(info:ProjectInfo) extends BasicProject(info) with JettySupport {
	val webResources = List("src/main/webapp")
	val webInf = "WEB-INF/web.xml"
}




// ============= Start JS Plugin Code =============

trait JavascriptSupport {
	val jsSources = List("src/main/javascript")
}

trait QUnitSupport extends BasicProject with JavascriptSupport {
	val qunitSources = List("src/test/qunit")

	def qunitRun = {
		print("Running tests in " + qunitSources + " on JS in " + jsSources)
	} 
}

trait QUnitSupport2 extends BasicProject {
	// Explicit Self Typed Reference
	// http://www.scala-lang.org/node/124
	self: JavascriptSupport =>

	val qunitSources = List("src/test/qunit")

	def qunitRun = {
		print("Running tests in " + qunitSources + " on JS in " + jsSources)
	} 
}


// ============= Test Code =============

class MyProject(info:ProjectInfo) extends BasicProject(info) with QUnitSupport {

}

class MyProject2(info:ProjectInfo) extends BasicProject(info) with JavascriptSupport with QUnitSupport2 {

}

MyProject implicitly gets JavascriptSupport added to it, while MyProject2 needs it explicitly added. This seems to be because "with" (also "extends"?) in a trait will implicitly add that to the inheritor of the inheriting trait, while explicit self types will not add it.