How can I augment NODE_PATH for renderer processes?

I’d like to open a new BrowserWindow with Node integration, and for the NODE_PATH to be augmented so that require will look in custom places.

How can we augment NODE_PATH of a renderer process or new window?

Note, I want to augment NODE_PATH without modifying renderer code. The renderer code should have no idea NODE_PATH was modified.

EDIT: I’m running the karma command, which is using karma-electron to run Electron. Setting NODE_PATH for the karma command does not propagate into Electron, so I’d like to figure how to do it in Electron (without modifying renderer code, if at all possible).