Okay, I don't yet know if this will actually work (ask me in a year!) but I've found this amazing plant called Rubus chingii var. suavissimus, or Chinese sweet tea. It's a close relative of raspberry whose leaves contain a substance called rubusoside, which is similar to steviol glucosides. So no calories, and extremely sweet. I've got a plant, ordered another, and am trying to grow some from seeds as well (diversity!)
The plant is supposed to survive temperatures down to -25 degrees C, and even if the tops are killed off, it's not necessarily a huge deal, since it works like raspberry in that each cane only lives for two years even though the root system is perennial.
I haven't really used the leaves for much yet, since I don't want to slow the plant down too much, but I did taste the leaves directly and made tea from them. Both the leaves and the tea taste extremely sweet, and without that "off" taste that I get in both stevia leaves and steviol glucoside based sweeteners. As a bonus, it's supposed to have a whole lot of good medicinal benefits, including reducing insulin resistance! In short, from everything I've read and seen, it seems awesome. The only remaining question mark is whether it will actually survive our winters.