Can a mounted object have different collision properties?

I've got an enemy that I want to collide with the player. This is working. That sprite also needs to collide with the platforms in my level. This isn't working.

I can't use the enemy collision poly for detecting player collisions when I check for platform collisions. I change that poly based on the direction the enemy is moving in, so I get some odd collision results. So I figured I'd mount a dummy object to the enemy, and then use that to detect the platform collisions. Unfortunately things don't seem to be working as expected.

Here's the code:

$vulture = new t2dAnimatedSprite() { scenegraph = t2dScene; };
	$vulture.setGraphGroup( $enemyGroup );
	$vulture.setLayer( $enemyLayer );
	$vulture.setCollisionMasks(BIT($playerGroup), BIT($playerLayer));
	$vulture.setCollisionCallback(true);
	$vulture.setCollisionActive(true, false);
	$vulture.setImmovable(true);
	$vulture.setSize( "64 64" );

	$vulturedummy = new t2dSceneObject() { scenegraph = t2dScene; };
	$vulturedummy.setGraphGroup( $enemyGroup );
	$vulturedummy.setLayer( $enemyLayer );
	$vulturedummy.setCollisionPolyPrimitive(8);
	$vulturedummy.setCollisionMasks(BIT($platformGroup), BIT($platformLayer));
	$vulturedummy.setCollisionCallback(true);
	$vulturedummy.setCollisionActive(true, false);
	$vulturedummy.setSize( "64 64" );

	$vulturedummy.mount($vulture, 0,0, 0, false, true, false);

With things like this, the vulture will happily fly through the platforms. Collisions with the player work fine.

However, if I change the dummy code to read:

$vulturedummy = new t2dSceneObject() { scenegraph = t2dScene; };
	$vulturedummy.setGraphGroup( $enemyGroup );
	$vulturedummy.setLayer( $enemyLayer );
	$vulturedummy.setCollisionPolyPrimitive(8);
	$vulturedummy.setCollisionMasks(BIT($platformGroup), BIT($platformLayer));
	$vulturedummy.setCollisionCallback(true);
	$vulturedummy.setCollisionActive(true, false);
	$vulturedummy.setSize( "64 64" );
	$vulturedummy.setPosition("0 0");
	$vulturedummy.setLinearVelocityY(10);

//	$vulturedummy.mount($vulture, 0,0, 0, false, true, false);

Then it drops down and lands on a platform.

It just seems to not work properly when mounted. Can I not have mounted objects colliding with different groups and layers than the parent object?

If not, any suggestions as to how to achieve my desired result?