I know you can Sneak attack Dragon with a Bow, but I'm not sure that it's possible to Backstab Dragons...
I could be wrong though. I just know that every time I've tried it with a Dragon, once I get within a certain distance, they take off whether they've seen me or not. My Sneak "eye" will still be completely closed, but the Dragons take flight everytiime and immediately attack.
My Sneak score is 100 also, and I'm wearing the Ancient Shrouded Armor. So I should be sneaking up on them with no problem, I just don't think it works with Dragons.