Just put %_topdir /some/directory/you/can/write/to %tmpdir /some/temporary/directory/you/can/write/to to your ~/.rpmmacros and create the appropriate SOURCES, SRPMS, SPECS, BUILD, etc. directory structure in /some/directory/you/can/write/to
If you use Fedora, there's a package that supplies a command to do this: <http://fedoraproject.org/wiki/Docs/Drafts/BuildingPackagesGuide?highlight=%28package%29>