Nick, thanks for validating my pain. :-|

In the meantime, I found out Python already knows more finely what kind of expression something is, and uses that knowledge in the error messages. Look:

    >>> a + b = 4
    SyntaxError: can't assign to operator
    >>> a(b) = 4
    SyntaxError: can't assign to function call
    >>> 'a' = 4
    SyntaxError: can't assign to literal

So, how about "keyword argument name cannot be an operator / function call / literal"? (Of course, if you ask me, "operator" is also unfortunate, but at least there is a precedent.)