I am reluctant to install from a "DESTDIR" using "build.sh"  on a
  running system, because I have the impression from the docs that
  this would overwrite things in /etc and elsewhere, unlike the
  old "make install" from /usr/src.

The build.sh install target does not write into /etc, just the rest of
the system.   That's the normal way to do an in-place from source
upgrade.  usual warning about updating kernel first, of course.


Also, you might find pkgsrc/sysutils/etcmanage of interest.

-- 
    Greg Troxel <gdt@ir.bbn.com>