The last "official" Open Watcom release (1.9) is indeed quite dated.  The codebase was forked for a variety of reasons where development continues: 

https://github.com/open-watcom/open-watcom-v2

The current development release does indeed build on x86_64 (the build system is quite complicated).  I'm running Open Watcom nightly builds on Debian Wheezy x86 myself.  I've mentioned supporting OW in Python, most notably in a PyCon lightning talk last year, but there really seems to be no interest even if there are some benefits.  OW allows building the full interpreter and standard library on Windows, for example, without any MS tooling.

I was using it in this particular case as a benchmark for compile times for a presentation in a few weeks.  OW is able to build the interpreter in ~25sec whereas GCC is taking 1min 10sec on my desktop.  I don't have any plans to pursue Python support for OW any further, although I'm open to the idea.

This patch is enough to compile *this portion* of Modules/posixmodule.c with OW, but, no, it is not sufficient to compile the whole file.  There are two more places where some preprocessor definitions cause issues.  One is OW-specific, and I don't plan on filing a bug against it.  The other is #22579.  I only filed this bug because the code was broken.