Thanks for the inquiry. However it's not currently our intention to design camera specific plates for the D5200 and without one in house to perform tests I wouldn't be able to guarantee a fit of the D5100 plate on the newer model. All indications are the body has changed enough the D5100 L-Plate wouldn't be an appropriate fit.
If you send me pictures of the bottom and side profiles of the D5200 lined up next to a metric ruler I'm sure we could find a conventional plate that would mount (no L-Plate, though).
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.