I also literally know no SCM, ever, that was not maintained on itself, except insofar as we might be able to have an epistemological discussion about early versions of CVS v. RCS. Bitkeeper, Mercurial, Git, Subversion, Perforce, tla, Monticello, Bazaar, darcs, Fossil, Monotone, and TFS are all maintained on their own systems. Subversion maintaining itself on Git would not exactly be a vote of confidence for me.
There are a couple of SCCS projects which are just barely outside of your epistemological discussion. ;)
CSSC, which is the GNU project's implementation of SCCS. It uses git: http://www.gnu.org/software/cssc/ . CSSC is meant as a stepping stone to migrate existing SCCS projects to some other version control system.
Sun released SCCS in 2006. A couple of forks exist. One is the Heirloom project. If I read http://heirloom.sourceforge.net/devtools.html correctly, they maintain their version of SCCS using CVS.
It would at least give you a lot of confidence that the decision-makers understand that there are differences between various SCMs and that there isn't a 'one-size-fits-all' solution.
More seriously, I believe that SourceSafe was maintained on Microsoft's fork of Perforce, but I don't honestly know, nor do I have contacts at this point who'd be able to find out.
I would surmise that the original authors at One Tree did maintain it on itself, but it's a pity that it's nigh impossible to find any sources on this.
