Just add a little (python?) script in the repository root that sets everything up; then in the checkout instructions tell people to run 'python setup.py' to ins

Just add a little (python?) script in the repository root that sets everything up; then in the checkout instructions tell people to run 'python setup.py' to install the extensions needed. I don't think you need to add VCS-level support for this; it'll just create a more complex program which can result in nasty exploits without having too many benefits.