Here's another possible mechanism:

def runtime(f):
        """Evaluates a function's annotations at runtime."""
        annotations = getfullargspec(f)[-1]
        @wraps(f)
        def wrapped(*args, **kwargs):
                defaults = {k: eval(v) for k, v in annotations.items()}
                defaults.update(kwargs)
                return f(*args, **defaults)
        return wrapped

@runtime
def example1(x, y:'[]'):
        y.append(x)
        return y

@runtime
def example2(x:'a**2+2*b+c'):
        return x

Pretty simple, although it messes with the call syntax pretty
badly, effectively treating a non-keyword argument as a
keyword-only argument. There's probably a way around that
but I doubt I'm going to see it tonight.

The point is, I don't really see the point in adding a new
syntax. There are *lots* of incomplete solutions floating
around to this issue, and it will probably take a lot less
work to make one of those into a complete solution than it
will to add a new syntax, if that makes any sense at all.

Also, do you mind posting any problems you find in that
to the activestate message board so there is a record
there?