A good sensative horse that has been well trained will definately lunge off of cues so subtle as eye movements, yes, I've seen it. It was quite amazing what this person could do with her horse; she never had to say anything to him and hardly moved...never gave "visible" cues for anything...
Lunging, especially free lunging, you are utilizing the horse's flight instincts and the behavior of a predator or dominant horse to get your horse to move away from you. If a predator is staring/ squared up with a horse in the shoulder or eye the BEST thing for that horse to do is turn around and run the other way (so this is why stepping sideways/forward towards the shoulder causes the horse to turn in and around). If a predator is farther back and squared up at/starring at the horse's flank the best path for the horse is to run the direction they are going because they are ahead of the predator (that's why we want to stand at the horse's flank to send them forward on the lunge).
Also, humans have this funny way of orienting their bodies where their eyes are. (notice when your riding and your eyes are looking at your horse's withers your shoulders drop too even when you think you are being sneaky and just looking down??? ;) ) So, if your eyes are on her flank then your shoulders are centered towards her flank, your feet are pointed towards her flank, etc (more than likely, unless you are making conscious efforts otherwise). This is where you want to be, body language wise, to send a horse forward. Your horse may or may not notice your eyes, but she WILL notice your shoulders and overall body position and whether you realize it or not your eye position effects all of that.