I agree on getting the advise from the owner, as on how they lunge this horse
I don't use side reins, as I lunge using my western saddle, just check the reins back
I only apply stud shank when the horse offers to try and pull away, and when I'm still lunging off of the halter, as opposed to the inside ring of the snaffle.
I ride both the rein contact, as in HUS, and also while schooling a western horse.
I never want a horse leaning on the bit, in either discipline. By slack, I don't mean a huge drape in that lunge line, with that line flopping around, just no constant pressure by the horse, leaning on that aid
I would also deal with the pulling, before worrying about any head set, thus the horse would be checked back with slack in the reins, just enough so that he can't put his nose on the ground, if I had the stud shank attached running under his chin
The idea of feeling 'trapped' occurs when someone does not understand truly how the chain under the chin works, hooked up correctly. Unlike a chain wrapped around the noseband, there is zero pressure applied by that chain when the horse is giving correctly and not pulling. There is no constant action of that chain, as if it is applied correctly, and is long enough, it releases and is non functional, unless the horse pulls , thus is being resistant
But certainly, the owner should be consulted, as it is not the OP's horse http://www.bing.com/videos/search?q=...ail&FORM=VIRE6
Maybe not the best example, but it does show what I mean by slack in the lunge line
another one http://www.bing.com/videos/search?q=...ail&FORM=VIRE6