It would be nice to have support for something like "/usr/lib/rpm/macros.d/", where system packages could store their macros files. See [1] for the discussion. [1] http://lists.fedoraproject.org/pipermail/devel/2012-August/170533.html
Support added upstream in rpm >= 4.10.90, the directory actually now exists too in >= 4.10.90-0.git11989.3.fc19