Not sure how there could be considering what the subframe connector does. Hard to imaging obtaining the level of rigidity required for tying the unibody "frame" together with bolt on connectors. Maybe there is some company that offers something but I'd be a little skeptical.
Weld-ins are the same as bolt-in from many manufacturers. It's just the cost of hardware that makes them different. My instinct is the holes drilled through the frame rails weaken the frame more than weld-ins do.