My impression is there are regs against warp in a system due more to traffic control issues. Figure Earth is super busy as a port of call, with Luna, Earth, Mars, Juptier, and other areas constantly having ships, shuttles, and what have you running about. You don't want to be moving at the speed of improbable, with a nanosecond to turn, should some drunk libertine in a space yacht, out for a good time, decides to pilot in front of you.
I would think you could get closer on warp in systems where the traffic is lower. In some places, if there's no traffic at all, you could cruise right up to a planet, lock up the brakes, and spray Cherenkov radiation all over the planet's ionosphere (Look at the pretty lights!)
I would say that getting within the corona of a star should act like long range from a cluster on the ship; higher it you're in closer or dealing with a larger star. Hit the actual atmosphere of the thing, I'd do damage as per the superjovian close approach, ratchet the damage up a die for every class of star above M (ex. 3d6 for a G class, 5d6 for an A class -- fiddle for giants, dwarfs, or what have you.) Just a suggestion.
Id also say the event horizon of a black hole, for a starship moving at warp, would probably be the point of destabilization for the warp field. But with a good engineer and pilot, you could conceivable keep the warp bubble stable enough to pop a little way inside the event horizon and have a look, since you could escape. Of course, you should do mega damage to the ship, but if still under warp speed, you should be able to escape...but into what time, dimension, or alternate universe..?
"War is an ugly thing but not the ugliest of things; the decayed and degraded state of moral and patriotic feelings which thinks that nothing is worth war is much worse. A man who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself."
John Stuart Mill