A plugin can expose it's api to the scripting DOM, the idml DOM or both. While the scripting DOM and the idml DOM are closely related, they are not the same, so maybe the plugin does not expose the properties to the scripting DOM, just the idml one.
If the plugin exposes some api to the scripting DOM, then, ofcourse you can use that. But, the plugin needs to be installed and activated on the machine you are running the script.
So the plugin needs to explicitly expose their custom objects to the scripting environment, otherwise other scripts can't access them.
Do you know where I can find documentation on how this is done from a plugin development perspective?
Szahnje, from your writing it is unclear whether you understand that there is a difference between "plugin" and "script".
Plug-in means something written in C++, like the original InDesign modules.
Plug-ins also provide supporting data in "resources", the relevant resources that end up in the scripting object model are called "ScriptInfo".