[ubuntu] GLX software rendering on headless Ubuntu 14.04

I have a headless GLX/OpenGL rendering server that works great with GPU hardware using proprietary drivers (nvidia-340).

I want to be able to disable hardware GPU rendering and do some temporary software rendering (Mesa) for benchmarking purposes.

However, everything I've tried kills X/GLX rendering - with various error messages, depending the approach I take. I've tried nouveau drivers, I've tried building Mesa from source, etc.

Can anyone point me to a group/resource that has working experience setting up GLX software rendering on Trusty?

Thanks!