I may be missing some important detail of the implementation, but it sounds almost like something that could be handled through a Usermin module or modules. Would you be able to run stripped down webmin/usermin on the servers?
Yes I can't publish all details at this stage. usermin does look interesting but I am not yet sure if it's right for this project or not - will have to advance the design phase further first, because that kind of decision is all or nothing for a project's use of it.