Explain your idea

The lanterns currently are dumb and have placement restrictions that don't allow the hammered part of a post to attach to them. All that placing a lantern below the post would do is cause it to flip down to connect to the lantern like any other block.

I've investigated this a little, and while I can allow the lantern to be placed below the post, I can not make the arm of the post flip down to meet it.
The reason for that is that the actual bounding box for the lantern is only in the center. The chain above it exists only in the model, and there is no indication in the code that the lantern even reaches the "ceiling" of the blockspace it is placed it.
There is no way to know that lanterns are designed to connect to blocks above them.

So while it's possible to make the placement happen, it just looks really stupid: