I think the piloncillo will have some nice flavors left over. Maybe a maple flavor, or some caramel.
Back-sweetening will add sugars, sure, but will you be happy with the body? That's one of the differences between cider and graff. The malt technique will leave more unfermentables, so it should give a bit more -- I call it "roundness", I don't know what the official word for it is -- to the palate.
Seriously, though, that is a sweet ingredient. You get kudo's just for using it, and it sounds really cool just saying it. "Here, try some of my piloncillo graff." Nice.