There is a new mock rpm for testing here:
http://cbs.centos.org/repos/core7-extras-common-testing/x86_64/os/Packages/
mock-*1.2.17-1.el7.centos.noarch.rpm
This mock has centos-<ver>-<arch>.cfg files that should work for building against all the released trees for CentOS-7 (x86_64, i686, ppc64, ppc64le,aarch64,armhfp), CentOS-6 (i686,x86_64), and CentOS-5 (i686,x86_64).
If these mock configs work, we will try to get them rolled in upstream in EPEL.
Please report any issues on this list.
Thanks, Johnny Hughes