Problemns linknig a Pyramid shape to a multibody

I have made some kind of distance sensor for my walking bot. For seeing the direction of the calculated beam I gave the thing a pyramid shape. But I have problems linking it to the Multibody the way I want:

Initially the Sensor (subclasses from Link) get rotated by 180° to face towards the ground and attached to the MultiBody via FixedJoint. I thought it would be a good idea to disable physics for that Link so that the sensor does no collision detection at all. But it seems that physics are needed for a proper link connection.
So enabling physics again but then the initial rotation of the link (which is formulated in the sensor's init method via "self rotate around-axis") is ignored. I have to set this externally in the class of the MultiBody.
Is there a way to fix that and perhaps is it possible to link the Sensor but giving it a non-collision body?