Very interesting. I wonder if this approach could be used with other tropical fruits. Though one would need to identify a graft-compatible species with notable frost hardiness.
I’m not sure on the graft compatibility but Diospyros and Persea both have temperate and tropical species and edible fruit. Ficus could be another although the most edible one is the hardiest in that genus.
Morus is another although there is minimal difference in fruit quality between species from what I’ve seen.
True. Most rosaceae are already hardy but maybe hardier peaches or almonds could be worthwhile.