What would be ideal is a canned setup and set of makefiles to build a fresh
system rooted in some alternate hierarchy.

Once you bootstrap the process, all you need is /usr/sbin/chroot.  If
you can populate a build target area with a compiler, a set of
libraries, and a set of include files, you can then build everything and
install it using "generic" rules.

You can even use null mounts and/or union mounts to facilitate the
process (by mounting /usr/src into the new rooted tree so you can build
in it, and/or by union mounting your bootstrap tools & include files
into the target area before you replace them in the top union layer).

Now, I don't have time or disk space to cobble together such an
environment, but maybe somebody here does?

==John