RFC: working with sparse work-copy: where to start?

Hi! I've been thinking about working in a sparsely-populated work copy. E.g. svn will let you do a checkout of a specific sub-directory and do all your development there assuming you only care about changes in that directory. Ideally, it would be possible to specify a set of directories that I care about.

git does close to what I want if I do git-clone -n and then git-checkout <subdirectory>.

However,
1. This re-creates the same directory structure that the repository has.
   Especially for when this directory resides deep in the original repo
   tree structure, it would be nice to be able relocate the directory anywhere.
2. The rest of the tree is checked-out whenever I pull from the central repo.
3. git-commit would list all files I didn't check out as removed.

I wonder how hard would it be to add such support? I tried looking at 2 - is builtin-merge-file.c the only place where the checkout is done on merge?

What other commands need to be fixed for this to work?