How much weight can you comfortably negative rig with this?

When it comes to regative rigging you need to consider the potential shock loading that could happen. This increases the force on the rigging hardware. This pulley has a working load limit of 20kN which is 4,496lbs.

So as long as that force (added weight from any potential shock loading) is under that working load you could consider that ok, theoretically. However there might be better alternatives for rigging hardware that takes continuous negative rigging.