I actually do this with one project, with a very small script that wraps cargo and sets $CARGO_HOME to a project local path before actually calling cargo. I keep a very short cargo.py and cargo.sh with my project.

I've experimented with using this to "vendor" dependencies in a local .cargo and so far that seems to work. An early example with discussion of the python version is here: https://www.reddit.com/r/rust/comments/3ea6je/is_there_a_bet...