Ok, it's locked, but then, just:
- set the variable to None as default
- after the code for setting the variable from environ (done by default on kivy, if you would set USE_OPENGL_ES2), do `if c_options['use_opengl_es2'] is None: ... set to True`

And that's all. It would take the environ value first if you set it, or use True if nothing is set. No need to add a new environ variable :)