"I'm mostly interested in the ornamental aspect of these large plants for my garden."
I'm still trying to think of large interesting plants you can grow from something in the 'food aisle' or non-seed packet source. Hybridization, I think won't matter much in the look of the plant.
You mentioned pumpkin seeds and hulled sunflower- sounds like those are processed for snacking, so probably no. But any whole hard winter squash should have hundreds of mature seeds, probably will grow a rambling vining plant with big leaves. Ditto fresh watermelon with seeds.
As mentioned, birdseed should grow. But depending on your choice of shopping, if you have to buy a bag of pounds of seed just to grow a spoonful- you may as well go get packets. Unless you want to feed the rest to birds.
I haven't gotten mango to grow but I cut the pod open once and the seed inside already had a root sprouted, curled up.
Other foods, not seeds, in the produce aisle can be a cheap way to grow - I have grown elephant ear type plants, sold as yautia and edo, they are edible roots.
