I not agree with you
- gpo require Packager : No, and gpo is not the unique way to install
- less concerned : if you use OCS, it will be for years, so you WILL be concerned in upgrade agent (and may be certificate) : Packager could be used for initial, NOT for upgrade
- psexec don't suit : No, psexec is very simple and efficient, it's easy to deploy from a server, or manually or by batch for a list of computer
I worked for a company who stay his OCS server and agent at 2.0.5 (=2012) : nearly 9k computers contact server each days. And I did the job to upgrade the solution : I breaked lot of (false) ideas. (And not only for Windows Agents !)
Try script and psexec ...