With Python, which has the easy_install utility for installing its own packages, I use the virtualenv mechanism to create a "shadow" environment connected to the machine's main Python but with its own library/package folders.

With that, I am free to install anything I want in the secondary Python, I still have access to distro updates for libraries that I chose not override in my environment and I am completely cool I am not hosing my main Python install.

In the unlikely event there is nothing similar to it for Ruby, it would be trivial to develop it.