What is the python-style equivalent of virtual environments but for a user account on a Mac OS?
If you are dead set on that paradigm, then virtual machines seems to be a logical next step, no? Grab Virtualbox and do your thing in a VM. 

Note however that package managers solve many problems virtualenv addresses. The libs you mention won't change interfaces radically, they are stable releases. 

It's python libs which are the most volatile, not things like FreeType. I would just use whatever version X11 is using.