It would be more pythonic to use an empty def call instead.
No, it won't. Python draws a very strong distinction between expressions and statements. This line has been blurred somewhat with the advent of comprehensions and the if-else expression, but it would still require more benefit than three characters in a keyword saving to allow def use in both statements and expressions.
The following, for example, does not look pythonic at all:
def transform(seq, func=def(x):x):
...
(Note that I attempted to emulate syntax highlighting to make my point.)