i don’t think this is reasonably feasible. it sounds like what you’re trying to do is have an overlay on top of an existing game but have all controls passed through. look at programs like nvida shadowplay, xfire, or the steam overlay. i’m sure there are others. maybe look at DX hooking programs like ninja ripper.
if you were to do this using UE4 you would need to run the program in the background, get it into unreal (you’d need a separate program for this and it would introduce lag, particularly bad for VR), you would need unreal to render your added elements on top of the program and receive input, and lastly pass that input back to the original program. this is inefficient, introduces lag, and would have to work with each program you needed. this would be a big project without trying to use UE4 and i’m pretty sure that you would have to write custom UE4 C++ libraries to do it; you might as well write those libraries as a separate program.