Yes the Vario case for the liquid cooled GS fits straight onto the RS rear rack.
It does not need an adapter plate or any modifications to rack or to the box.
There is also a 49 litre BMW topbox that fits. According to the label inside it is intended for the XR and GS but will fit after removing the sticky out bit off the base plate. Some dismantling of the box is needed in order to access two screws and remove the sticky out bit. After that it fits straight onto the RS rack. Suits the RS lines far better, in my opinion and I suspect it will be more pillion friendly.
Pic attached of my bike with the 49 litre box and a screenshot of the other guy’s RS with Vario.
There are threads on here (RS section) showing both types of box.
Here’s a couple of pics , showing both types, to be going on with.
The locks in both types can be reset to match your ignition key.