Patch management on multiple systems

It is common to have some sort of proxy cache service acting as an intermediary between the upstream repositories and the target machines. It helps to save bandwidth and makes deployments faster, in the RHEL/CentOS world that would be spacewalk, in Debian and derivatives a debmirror and in the AIX world possibly a NIM server.

Given the absence of ruby for AIX (some ports do exist, though), the appropriate configuration management for your environment would be cfengine. If that looks overkill, you can also try ansible, which only requires python. And there is python even for AIX.

It is also common and even expected that the versions, name of the services, name of the packages, etc... differ from platform to platform. You need to deal with separating data and code. If a mature configuration management system does not meet your requirements, I seriously doubt that a bunch of home-brewed scripts will.

The third requirement: "I can't install anything on the target system..." is not clear at all, and seems to be in conflict with the fact that you are using private repositories.

Last but not least, managing packages is only 1% of the tasks you need to perform in order to secure and audit your systems.