I am not quite sure this will be a best practice but in my opinion, this approach could be achieved.
One thing is, during the installation of the Update Manager, you need point to a vCenter server. Probably you have a standalone ESXi 5.x box which isn't registered/managed to/by one of the vCenter servers?
Please keep me updated how it goes, this is quite interesting
As stated, you need to have vcenter for running update manager.Update manager is a windows application thats installed normally on any windows machine. Also Update manager doesnot download patches on the system which it is installed. It downloads only the metadata of the patches release from the download source. VUM scans the host with the patch metadata and check which are compliant patches based on which, once we click on remediate, it starts downloading the patches onto the temp location inside the host and installs it.