Knowing the lunge on bit only was over in his mind, I increased the effort required on Harley's part...
At the arena, I added gloves, his "baby bit" french link snaffle, and surcingle, side reins attached to it (but not the bit yet). I lunged him just a little on the snaffle only, and after he fought it just a little while, he realized it was much like the halter lunging. Same, but different.
After a few minutes, I added the side reins. Like it never even happened, Harley pressed forward, and lunged out. Leads me to believe this isn't his first experience with side reins, or something like it. He did leave his mouth gaping open to begin, but then settled easily.
I tightened the side reins once, looking for some sort of "mental feedback" from him. No reaction. No wrinkled eyes, no flappy ears, no stressful gaits.
Total work about 30 minutes. Neither of us broke a sweat. How cooL!