As I'm thinking about the OP's question, a couple other games come to mind. Granted they aren't WoW, but the idea of a ranged sneak attack was used in AD&D 3rd edition, where a scoundrel could add his/her sneak attack bonus damage to the ranged attack if certain criteria was met. The other game was the video game Thief, where if you were in unoticed and hiding in the shadow, you could effectively snipe someone with a bow and arrow as if you had snuck up on the mob and knifed it. Again, those games are not WoW and the tool tip for Ambush specifically says requires dagger, but I can kind of see where the OP might have been coming from.
Although, could you imagine being able to Ambush from range, get a couple of more shots in, having a couple of combo points on the mob, and having a full energy bar by the time the mob got to you? Smells like a win/win situation that woudld be quickly nerfed.