Also, it's an Ur-Quan forcefield and not a Precursor one, so they aren't necessarioly that alike. Ad if you bring down the shield, EarthGov is going to take over. It'd be a practical idea in a sequel though. At that point we know we can bring down shields, since the Chmmr do it, and exploring the galaxy we're bound to find many slaveshielded planets...
the precursors are smart it might be in there computers. ways to make them ways to turn them off
they might have wrote the book on shields and forcefields