Hello Sebastian,
Of course you can offer a wiki even outside a course or group, like any other content object, e.g.: the Feature Wiki at www.ilias.de. This is exactly the scenario you describe above. No plugin or patch needed. You only need to configure the permissions correctly. Anonymous needs VISIBLE and READ permissions so users that are not logged in can access the wiki. Logged in users (role USER or how it is called on your system) shoud get additional permission permission to EDIT CONTENT.
Have a try.
Kind regards, Matthias