Hello Jonny,
W#e did state it up front ... no changes to the SRPMS except for trademark changes.
This is a good decision, but then you could also publish the list of changes you do e.g. within the build environment to let other people also work within the same environment. Duplicating the efford done from RHEL -> CentOS to each developer out there who e.g. wants to test a patched src.rpm keeps CentOS smaller than it could be.
On the other side also the page from SL at http://www.scientificlinux.org/distributions/6x/build/ shows that this is not "completely solved" within a script until now and needs further development work.
regards,
Florian La Roche