Manipulating gravity on such a scale isn't exactly a small feat. We know the Vishnans use very strong gravity forces instead of magnetic bottles for their beams, and Knossos portals probably manipulate gravity to some degree (we know gravity is a major data in subspace theory). We know the GTVA and the Feds master gravity to some degree (Sol Portal, intra-system jump gates, gravity aboard ships), but it's not exactly an energy-cheap technology (we know UEF capships use rotators to save reactor power).
Since intra-system subspace requires a gravity field to operate, you'd need to cancel the gravitational interactions on a specific ship to prevent it from entering subspace... in theory.