As Jean-Paul mentioned, there is a ticket which almost exactly describes your idea: #1490. In particular, you can read my last comment explaining what that ticket is supposed to be about:
http://twistedmatrix.com/trac/ticket/1490#comment:39
If you'd like to implement what's specified there, we would be happy to receive your patch :). Making 'twistd foo.py' equivalent to 'twistd run foo' is a trivial afterthought once #1490 is implemented.