From memory, I don't think you can. However you can place an object (like a line) at the top of the subform and access that's y coordinates.
Thsi example http://assure.ly/uPXhPk has some examples of moving objects around a form, based on x/y coordinates. This example http://assure.ly/pGOS6t has script to maintain an object in a set position. I probably have some more examples.
Hopefully these can help you,
You can use the xfa.layout.y method, but if you have just added the subform then it will return zero until the subform has been rendered, so you may have to move your code to the layout:ready event.