I might have a suggestion. FTL commonly works with Nav probes whhich broadcasting their position in an orbit around a sun. These probes, while made by several factions all use the same broadcasting protocol (part of the FTL sharing agreement). Most probes can also broadcast data about the star they orbit and its system (if they are big enough). Probes who provide this info orbit 'known systems' and therefor are deemed safe. Some smaller probes set out further into the dark void. They can broadcast their position but no info. These are called 'dark systems'. So explorers will have no idea if the system is safe or if the probe simply got caught in the orbit of a black hole. Beyond that, there is Dark Space. A place where no probes ever went. As a faction, you could mass-produce the probes and throw them into space. Hoping they get caught in something.

Inside the system, sub-light would be standard except in close proximity of orbital platforms, planets, moons, other celestial bodies. System juristriction would go until X (can't imagine a right number as of yet) kilometers beyond the largest orbiting celestial body.