If your concerns are driven by resource requirements, then I posit your concerns are driven by limitations of fully distributed version control tools of today. Shallow and/or narrow clone (like the Subversion model) limit the amount of data required on clients and thus facilitate monolithic repositories without the extreme resource requirements on clients.

I posit that if Git or Mercurial allowed you to clone a subset of directories, the differences between a monolithic repository and a set of smaller repositories becomes indistinguishable, as a clone of a sub-directory is functionally equivalent to a standalone repository! The problem is that narrow clone is not implemented in any popular DVCS tool today (but Mercurial is working on it).