As long as it would be feasible to serialize the unverified user contacts into a JSON within the payload size limits (5MB), it might work. However, there are other details from this question that are not mentioned here, that will also determine the feasibility of creating such a workflow. Specifically, it is not clear whether the push notification will need to be sent to each user in unverified segment, whether the push tokens exist in ACS etc. From External API point of view, the main points to consider would be primarily the size and format of the JSON exchanged with external CMS.
