How prevalent are hyperspace membranes? In my example I said they were created by the fusion reactions of stars. That means the smallest body that could generate one would be a brown dwarf (abut 12-13 Jupiter masses) that can fuse deuterium. In other words there could be rogue planets in deep space that are nearly unreachable using FTL. These planets might hold all manner of interesting people, places or things, just looking to keep a low profile.
What about white dwarfs, neutron stars and blackholes? Perhaps the membranes their reactions created could persist for millions of years after they died out. In the case of these massive objects the membranes might break the normal convention of brane sizes and be perilously close to the hyper mass objects. They might be in a state of flux as they slowly decay with no energy being put into them or pulsate, changing in size over days or hours, in which case a badly timed jump could put your ship in great danger.
As for multiple star systems, I'm inclined to say they generate one set of branes in the case of close binaries and separate branes if they are several hundred AUs away. I already made things complicated enough. On the charts I did so far it is possible to have a star's branes completely inside the branes or a single brane of a nearby larger star. In this case one hemisphere of the smaller star's brane will be active and ready to accept or emit starships. This makes things much easier for shady types going in.
Planets could have an effect on branes, I said gravity contracts them, remember? A large planet orbiting close to a brane could pull it one way or another, varying the radius by an AU or more. This too could mean timing jump correctly would shave a a few days off your trip.
Finally if the branes are with you you could use them to cut time off an insystem trip. A good navigator could jump from one jump point in a system to another in the same system. Again this makes it easier for naughty crews to stay ahead of the law. At the very least a ship going from System A to System B could arrive in System B, then jump ASAP to System B's jump point for System C and change its transponder to calm it arrived from System C. This is handy if System B has tariffs on System A or favored trade status with System C, or you just want to throw the bank off for a few months.