Replying to my own post (for reference):
--- On Sun, May 2, 2010 at 6:46 PM, Shakthi Kannan
| Is there something that
| rpmbuild can import in its environment before starting the build,
I just checked that ~/.rpmmacros (at user-level) does override macros,
and /etc/rpm/macros* for system-wide.
Sorry for the noise!