Just curious if sudo would work for your purposes with dbus? You can set up /etc/sudoers to allow passwordless access to dbus for your user account and limit it to dbus only:
customuser ALL = (root) NOPASSWD: /usr/bin/dbus-send
EDIT: Just to be clear, ‘customuser’ above would be the user account you want to give sudo access to. So if the username is ‘fred’, you’d replace ‘customuser’ with ‘fred’.
Finally, just so I understand your use case, are you doing an initial provisioning of these devices through resin.io while they are connected to the internet and then moving them to a private network?