This is why I try to buy my cameras as they are coming out with new models. Buying the older model saves money as they will lower the price to get rid of inventory and make room for the new model. Plus, since the camera one is buying has been on the market a bit, the aftermarket companies already have out their gear for these models.
I love RRS L-Plates so much that I would not even buy a camera that had no L-Plate option. I have one for both my D200 & D5100 bodies.
They may say they have no plans but if the camera sells well and they feel they can make some money, there's no doubt in my mind that they would come out with a version for the D5200 eventually.