It's easy to share run configurations instances in IDEA - simply instantiate a configuration and check "Share":

I'm already version controlling the resulting files in .idea/runConfigurations (in the relevant project) and part of ~/.IntelliJIdea* (for puppetising desktops). However, I can't find where IDEA stores the configuration defaults - it doesn't seem to be in either of these places. They must obviously be persisting it somewhere, because it works across restarts. The official documentation is unusually unhelpful in this case:
This check box is not available when editing the run/debug configuration defaults.
The particular use case is that I'd like all future "Behave" configurations to have the environment variable DISPLAY set to :1 to run browser tests in VNC rather than in the foreground.