That’s actually an interesting use case. I didn’t see previously that variable names can be dynamic in this plugin. The original assumption was that a plugin would register its tags and variables once in the “enable” call. But for your case this is not sufficient.
You have solved this nicely with the tools available, but it is still a somewhat weird hack where you need to construct the module name to unregister. I think we should make it easier to allow a plugin resetting the registered variables.
Also registering the same variable multiple times from the same plugin should not result in multiple entries (the multiple entry support was implemented, because multiple plugins could potentially register the same name).