Ok I have finished the patch, I'll commit it during the week

> FWIW I wrote a module that overrides the default build_py and sdist
> commands with versions that allow specifying package_data recursively

Maybe that could be a new feature ? 


> (while preserving file permissions, unlike the - buggy IMO - behavior of
> distutils) so that I can get rid of the MANIFEST.in.

Sounds like a bug to me, could you fill an issue on that ?