I guess the question is when you know the additional keyword argument.
If you already know what that extra keyword is when you are
constructing the form list, you could use a partial[1] to put in the
arguments you know already. Partials are python 2.5+ only, you can use
a lambda instead if you are stuck on 2.4. That doesn't help if you
need the kwarg to come at run time though.
[1]
http://docs.python.org/library/functools.html#functools.partial