Just a side idea, maybe what you want is not new syntax but a linter?
Knock together a script that runs through your code and tells you
about any oddities it finds, thus guaranteeing that your stuff does
indeed match up. As an added bonus, you could plug that into your
source control system so you get an alert before you can commit - not
sure how you do that in Mercurial but I'm sure you can (I've only ever
done it with git). Makes it really easy to catch problems.

I've thought about that but rejected it because then I'd have to change all these functions to be keyword arguments. In an example I just scrolled to randomly this changes a function call from 368 characters to 627! I believe there are several worse examples :(

With my suggestion at least that function call would only go up to 595 without changing local variable names. If I also add the feature to my suggestion that "foo(bar=something.bar)" == "foo(=something.bar)" (which is pretty horrible!), I can get it down to 500 and still use keyword arguments. And there'a few superflous Nones that I can get rid of if I use keyword arguments, but only 5, which in this case doesn't change much.

Writing these numbers gives me the feeling that it's indeed this code base that is pathological :P

If you know of an automated tool to convert functions from positional arguments to keyword arguments that'd be a fun experiment to run :P