We would like to be able to extract only src/sys or ports/french out of the repository for efficiency and disk space reasons.

This is an often mentioned feature which is not implemented yet. You can workaround this limitation for the working directory in Hg but not for the repository itself (.hg/*).

Git has recently gotten preliminary support for submodules which are linked repositories within a super-repository. This functionality may be too heavy weight for partial clones though, in which case a simple daemon could be implemented that checks out only a single tree object from a repository without history.