Here's a C implementation.  I'm a bit under the weather so please do double check my refcounting logic. ;)

No tests or docs yet, but those would be easy to add.  Here's an example:

@public
class Foo:
    pass

public(qux=3)

print(qux)

@public
def zzz():
    pass

public(jix=1, jox=2, jrx=3)

print(__all__)
print(jix, jox, jrx)

You could also try to add an explicit __all__ in the module and those names will get appended to it.