I'd change the name of the subdirectories of build instead of the name of the build directory itself. That way distutils will still create one build directory regardless of how many python versions you use, and the name of that directory is easily predicatble (which is nice when using basic tools to clean up a directory).

The distribute/setuptools projects do this as well, and implementing this should only require a fairly trival patch to distutils.commands.build (the defaults for the various build_ directories need to be updated).

BTW. How likely is the acceptance of such a patch? IIRC distutils is more or less closed for development w.r.t. new features. Would that also apply to this feature request?