Mercurial has extension support, and one of them (used for example on the OpenJDK project - a "real" project of some size) is "Forest", which does just that: http://www.bitbucket.org/pmezard/hgforest-crew/overview/

there is no documentation at all...

Most things can be found on the Mercurial repository:

http://www.selenic.com/mercurial/wiki/index.cgi/ForestExtens...

Not sure why there are no docs in the hgforest repository though - that seems like a pretty big oversight.