I am not familiar with the doings of meat packing plants, do they take live animals? How are they regulated? I do not know of any in this area. It sounds like it may not be a bad choice for an already dead animal.
I do know of universities that will take bodies for exam, necropsy, and then cadavers (sp) for students, and once sent a colt who had died to OSU under those circumstances. There was a charge for the necropsy, I did not pay for it and do not know the amount of the bill.
The term Kill Pen though bothers me, I do not know the logistics of them either; however, imagine taking a live animal. Receiving some small amount of money and leaving the animal to suffer waiting for a terrible long-suffering crowded road trip across the border to slaughter.
I apologize for my ignorance, but the first two options seem much more compassionate to a horse that has served you well.