I'm pretty sure the apple juice will ferment out, leaving very little sweetness. AND, if you add apple juice to any priming sugar, you could have boms on your hands. I had to throw a batch away earlier this year after priming with just apple juice - just miscalculated - and had several bottles explode.
I just back-sweetened a brown I just bottled using Splenda - I was kind of worried about using Splenda (actually Kroger brand), but my LHBS guy said he read a pretty extensive report that stated that Splenda imparted the least flavor of any artificial non-fermentable sweeteners (including Stevia). I'm still waiting on the final carbed version, but at time of bottling, it tasted pretty good.
