A heroku buildpack for setting the ssh private key as part of the application build. It's meant to be used with heroku-buildpack-multi, before other buildpacks which require the key to be present, like installing private npm modules from github.
Upload the private key to heroku (note that the key needs to be base64 encoded).
heroku config:set SSH_KEY=$(cat ~/.ssh/id_rsa | base64)
Add a .buildpacks file (used by heroku-buildpack-multi) which contains this and the default node.js buildpack.
https://github.com/debitoor/ssh-private-key-buildpack.git#v1.0.0
https://github.com/heroku/heroku-buildpack-nodejs.git#v75
Now as long as the public key is present on github and the user has the correct permissions, it's possible to install npm modules from private githup repositories.