Even so, your recipe doesn't quite work in several cases, aside from
when the source code is not accessible.  Two examples:

def nesting():
    default = 3
    @runtime
    def example3(x=default):
        return x
    example3()

nesting()

@runtime
def name(x=a):
    return x

name()

* The first one fails because default is not a global variable, thus
not accessible from within the runtime decorator.  I don't know how if
this can be fixed.  Note that for the function to exec() at all, you
need to e.g. modify remove_decorators so that it also removes initial
whitespace, something like:

def remove_decorators(source):
    """Removes the decorators from the given function"""
    lines = source.splitlines()
    lines = [line for line in lines if not line.startswith('@')]
    indent = 0
    while lines[0][indent] == ' ':
        indent += 1
    new_source = '\n'.join(line[indent:] for line in lines)
    return new_source

* The second one fails because of a clash of names.  I guess that can
be fixed by specifying what the locals and globals are explicitely in
the calls to exec and eval.