I think the problem is that no provisioning user is tracked in the database. Only the user who owns the task. I had published the deployment task for all provisioning users in the public tasks section. The user "public user" is written in the field "task_owner". When i copy the task in my own user task section, my username appears in the owner field and i could use it for my requests.
It would be nice if there where an additional field with the used provisioning user but that would be a feature request. Otherwise i must use a manuel action to provide the provisioning user information in the inventory or public variable section. This info however could change in the next deployment and must be provided with every new deployment. Thanks for all your information and i will use your MBSDK doc for future requests but this was a "nice to have" task an isn´t truly necessary. I thought that it would be an easy task. Request from the database or parsing the Info from a logfile but this information is not tracked.