I'm actually working on this very same issue for my wife tonight. I'm bottling my apple-cranberry cider and will backsweeten at bottling time.
What i've done is to take the 3/4 cup priming sugar and it was short about 1-2 tablespoons so on top of that i added 13 packets of a product called "Stevia In The Raw". The ingredients are quite simple: Dextrose, Stevia Extract.
Knowing how SUPER strong stevia is, i'm hoping this does the job. The 13 packets equalled out to about 2T, so if it's undercarbed, BFD. I don't think using 3/4cup of this Stevia In The Raw would be a good idea. Stevia can leave an aftertaste in high doses.