Havent seen any answers but was also interested as I am also modifying my urban lot considerably. . My thought is that Zanthoxylum americanum
(prickly ash) forms thickets and might be unsuitable for training. I too have been trying to figure out how to incorporate them for giant swallowtails. Large container maybe? On the otherhand, I have been toying with planting rue for the same reason. However, it is said to create skin rashes so I wonder how common that is.
(spicebush) branches well in all directions so it would be interesting to see if they could be trained. Seems like it might do OK in a pot though. I was thinking about trying it in a container as well. Of all the foodplants usedby spicebush swallowtail, L. benzoin
would seem the easiest to plug into an urban lot.
(red buckeye) on the surface might seem easier to train though I don't have firsthand experience with growing it that way. I planted it at my folks' house many years ago. It did well for about 15 years the succumbed to some sort of disease.