Re: Magic... but not Magic
Let's analyse how you want the power to manifest.
Using the example of a "Ghost Fence", there are a number of possibilities:
1. The barrier is really there, for the Tribal. It is something that only they can see, and therefore believe in implicitly. This could be modeled with the Entangle, with suitable advantages and limitations.
2. The Tribal can see the barrier, though it's all in his head. You could use Mind Control, or even a form of Mental Images, once again, only visible to Tribals.
3. The Tribal knows that the magic is potent, and, although the only visible markers for it are a few bones, and other items of power, he knows the great consequences for violating the barrier. This makes use of the beliefs common to all Tribals, and makes it an extreme sacrifice for the character to willingly violate the warding (read: Ego rolls at substantial penalties plus roleplaying required to break the "Ghost Fence").
Of these systems, I feel that #3 is perhaps the most elegant. It has definite roleplaying hooks, and creates a moody and mysterious system of "magic". It also covers the duality inherent in the system: that it only seems to affect Tribals. To enforce the "bad effects" of ignoring the magic, I'd probably use an Unluck dice system, by which Tribals would gain extra dice of Unluck for each violation, until an "atonement" ritual could be performed.
Of course, YMMV,
JoeG