I tried to apply @safe/@trusted/@system to Phobos. See also http://lists.puremagic.com/pipermail/phobos/2010-May/000488.html I found this issue there. Document says "Functions are overloaded based on how well the arguments to a function can match up with the parameters." But @safe functions can overload between @system functions, though @safe is unrelated to parameters. It's enough to have only @safe function.
Although I wouldn't regard it as a good practice to overload @safe/@system, I don't see much of a reason to disallow it. Overloading @safe/@trusted does not seem to be any issue at all.