Since you mentioned it, I Love Clicker Training!
If you're curious to learn more about the psychology and the ''hows and why's" it works check out this thread Clicker Training: Challenge Accepted
There's some great tips on how to get started and the basic principles of the training style.
The reason I recommend it so highly is because I use it myself on several horses, including a feral pony who sounds an awful lot like your mustang (this pony lived her first 15 years loose in a large field, never touched). She has come around a great deal, a 13 year old girl started her training (under guidance) and together they've come so far, unfortunately the pony is too little to ride, but they still have fun doing unmounted agility and other silly games :)
I also use it for several other rescues and even just to fix little issues with well trained horses, like my Belgian who just couldn't stand still for mounting, no he waits patiently until I ask him to move.
The biggest problem you'll ever find with Clicker Training has nothing to do with the training itself, it's all the crap you'll hear from other horse people. Don't let it dissuade you, the method is solid, just stick with it and prove them all wrong :) Only the CT thread you can also post your updates, we love to see how clicker trained horses are coming along!
Good luck and enjoy your lovely new project :) I can't wait for pictures!