vRO does not provide a scripting API to work with zip files (apart from the ZipWriter class which you already found out). And I'm not aware of a vRO plug-in that provides such functionality.
There was indeed a zipReader scripting object as part of the LivecycleManager plug-in part of the LifecycleManager product we got years ago.
I remember requesting this functionality to be merged in the platform since we already got the zip writer but it never made it. This could be a low hanging fruit that would avoid having to use commands on the appliance or on external hosts.
I finaly use a "command.execute" to unzip files, but it will be great if VMware add a ZipWriter function to vRO.
Thanks for your help