I have decided to release an in-house module that I have been using for my hosting company. A full explanation of the goals of the module is available in the README.

https://github.com/aschlosberg/ngx-shared-env

I am well aware that this module introduces (minor) additional per-request overheads. However, experience has shown them to be far superior to the alternative of Apache whilst allowing the freedom to run a shared environment without constantly restarting the nginx. As my knowledge of the inner workings of nginx is not as sophisticated as that of others I welcome feedback regarding optimisation.