I finally got CentOS 6.4 to rebuiild openssl with elliptic curves enabled, as required for Bitcoin.
The primary issue is that the CentOS openssl source package contains tarball openssl-1.0.0-usa.tar.bz2
The correct replacement tarball package from openssl.org would be this one http://www.openssl.org/source/openssl-1.0.0.tar.gz
Secondly, a patch from openssl.org is also needed: http://cvs.openssl.org/patchset?cn=19998
After the above two downloads, the spec file is edited to enable EC, point to the new source, and update the release number.
Detailed instructions on to do the build are here: https://bitcointalk.org/index.php?topic=239337.msg2624466#msg2624466