basedir is hardcoded into mock-helper so that people can't mount /proc whereever. But you can change basedir in the config file. We should either make it so changing it in the config file then lets things work or remove the option
I worked around the problem using a symlink, but an interim solution would be to produce a warning to minimize confusion.
I'll add a snippet of code to check that configs['basedir'] is still /var/lib/mock in the 0.6 code base. We've looked at this repeatedly and there's just no safe way to change mock-helper. So for now bind-mounts is still the only safe way to change where /var/lib/mock points. Shouldn't be an issue for the 0.7 codebase and up, since we removed mock-helper and the setuid launcher just runs python on /usr/bin/mock.py