It turns out that some kind soul added buildsys-build and buildsys-macros packages to the Fedora buildgroups repository. That considerably simplifies setting up a mock buildroot for RHEL, much simpler than my previous entry. All you need to do now to create a mock buildroot for, say, RHEL4 is:
Create a local yum repository with the RHEL4 rpms (as described previously)
Download the simplified [mock config file](http://watzmann.net/software/mock/rhel4/rhel-4-i386.cfg) and point the `[base]` repository at your local yum mirror.