Eggbutt bits are easy to longe with. The safest method is to run the longe line through the inside bit ring, over the poll, and attach it to the outside bit ring. This method of attaching the line must also be changed each time the horse changes direction. This method has a slight gag effect, raising the bit up and applying pressure on the corners of the mouth and placing pressure on the poll, but puts less lateral pressure on the bit. Helps there to be less tension, but lots of control. Do not attach the longe line just to the inside bit ring, as the outside ring can slide through the mouth when the line is pulled and damage the horse's mouth. Also do not run the line through the inside bit ring, under the chin, and attach it to the outside bit ring, as this causes the bit to pinch the horse's jaw, alters the action of the bit upward to put pressure on the roof of the horse's mouth, and because it causes more pain than control, the horse often resists the pressure and will not perform properly. You could have trouble with your horse locking its jaw when you ride if you longe the wrong way. Hope this helps, here are a couple pictures for you!
http://www.stepintodressage.com/images/lunge.jpg
http://www.stepintodressage.com/images/right_length.jpg
Here's the full site with that page of how it is supposed to be:
http://www.stepintodressage.com/inhand.html
PS: This isn't just a dressage technique, it aids with dressage. Hunter/jumper/eventer compatible.
Good luck!