According to this recent G Suite Developers Blog post, installing developer add-ons has now been enabled for users of G Suite.
And indeed, when I go to 'Settings' -> 'Add-ons' in my G Suite account, I do now see the 'Enable developer add-ons for my account' checkbox.
However, when I click this and enter my script's deployment id, I always get the same error message: 'Invalid Add-on'.
I would be interested to hear if anyone has got this working or not?
Edit 02/02/2018:
Further to the above and following communication with Google Support, apparently it looks like a setting in our G Suite Admin was causing the problem.
We managed to fix it by following the instructions here, making sure to select 'Allow users to install any application from G Suite Marketplace': https://support.google.com/a/answer/6089179?hl=en
It's a bit counter intuitive that a setting involving the Marketplace should affect our developer plugin that we hadn't yet put on the Marketplace, but in any case, after an hour or so our developer plug-in started to function.
Obviously it also is not ideal having to allow any app to be installed from the App Marketplace. Hopefully Google will put a more permanent solution in place soon.