The spatial integrity of the structure could be enhanced, perhaps, by using a ferromagnetic "ink" inside a structure that moves and varies the strengths of magnets to keep the material at the tip of the pen at its centre.
It's trivial to create a "magnetic well" with three to four strong magnets balancing against each other at a fixed point, with gravity holding a ferromagnetic object against it. Altering their directions and strengths would allow that single-point "well" to move just behind the pen. If the hardened material could withstand the force one would be able to prevent the drooping witnessed in the video. I'm not sure how one would maintain multiple wells simultaneously from a single plane. That or move the pen slower. Clearly, magnets win.
