In a few days, when 3.1 comes out, 3.0 will be mostly dead.
I think it would be more useful to put energy into a 3.1 port.

Other thoughts:

Your patch amounts to a in-development version of a new feature. To me,
that belongs on a separate branch or repository where those interested
can work on it. I am surprised that no one has set up a py-mingw project
on SourceForge, Google Code, or wherever, if indeed no one has. If not,
someone should. New modules from 3rd party developers are expected to be
publicly released for testing and use for some months, at least, before
adoption. Though it is not my decision, I would require the same of a
new port

I am not sure that minor-platform code like this belongs in the central
repository, especially when we transition to mercurial, where central
repository loses some of its meaning. To me, every ifdef from yet
another system makes the code harder to read.

So I recommend that you think in terms of maintaining a separate
Py-Mingw mecurial or whatever repository indefinitely as a public
project site where you and others interested can have your own wiki,
tracker, mail list, and release schedule -- all without permission from
the PSF developers.

I notice that your patch has a lot of FIXMEs. I presume that they only
apply to mingw, so that would be wrong for inclusion into the core. The
freedom to put platform-specific notes in the code is to me a reason to
have a platform-specific repository.