The joint where the main frame rail meets the vertical pivot tube may require a gusset or triangulation or something. With the top of the pivot tube unsupported, there would be a bending force there.
How about an upper hoop from the pivot back to the shock mount? That would limit what you could put on the cargo deck a bit, but how much bigger than an ActionPacker would anyone go?
You could even make it a bolt-on option...