Overall, the flow you described seem similar to Gluu's Resource Owner Passwrod Credentials grant flow. In this flow, you indeed POST some user's creds to `/token` endpoint, and receive an access token in a response. Though the user in question is expected to already be present in the system. I'm not aware whether it's possible to make it work with an empty password, and enroll user on the fly, but Gluu does offer a plug-in custom script allowing you to implement some custom logic for this flow, so it may be possible even without further customizations to underlying APIs (which is much more challenging and costly task).
As said before, if you are considering purchasing a support plan from Gluu to help with your integration projects, it's better to discuss it in a meeting, with responsible dev team members present and able to provide their assessment.