Yea, I was going to say you might want to look into Myler bits. There is one in particular (can't remember the name) that offers the most tongue relief. It's one that has the widest port, therefore offering the horse more tongue relief.
Single jointed bits apply pressure to the bars mainly, with *some* tongue pressure, but double jointed bits work mainly off of tongue pressure.
I'm just throwing this out there, but Parelli and Myler have just come out with a new bit they call the Parelli/Myler Cradle Bridle. Pat and Ron Myler developed a 3 specific bits for the Parelli program based on your horse's Horsenality. The bit itself is the same, but the amount of tongue relief is what differs. They don't have much info, out on it yet, but I believe they will start selling them at the end of this month.