I actually tried the same thing a while ago with no success (and knowing about form customization). I wanted a slider migx grid to only be shown for the home page, where it's used (nowhere else in the page, or if, I'd like to specify that later for specific resources in the same way).
So I set the restriction field (inside form customization rule) to id (which results in modResource.id) and the restriction to 1 (for resource with ID 1 = home) and unticked the box (under Template Variables) for the respective migx tv (didn't matter what kind of tv, also tried with others), then saved.
The outcome was, that the grid was now hidden on every resource, also on the home resource...so I'd never come so far to find out why this doesn't work...I also filed a bug here:
http://tracker.modx.com/issues/9012 but never got any comment or feedback about it. So now as somebody else has the same problem I would be curious about if I was doing something completely wrong or if it's really a bug =)