Well there is no such thing as an IK rig so to speak but rather a rig that has an IK solution applied A typical control rig would be a combination of different solutions that includes the ability to switch from FK to IK. High end animation apps like Motionbuilder, 3ds Max, and Maya have the abilities to create control rigs that offers function and features that offer a broad range of scripted options.
As I see it the current state of the UE control rig offers the building blocks to build the framework that will add flexibility of being able to use stock Unreal animations on any custom based rig with out having to retarget animations.
It’s not something new but has been around for a while, like in Assassin Creed 2 which uses full body IK, where the IK can pull the body towards a target.