I don’t think there way to do that without heavily modifying things in UEnigne class and RHI, engine module alone contains lot of 3D stuff… i mean this is 3D enigne so what you expect? 
Unity can do this thanks to it’s 2D enigne roots, Unreal since it’s origin of Unreal (the game) was always 3D engine. Not to mention UMG was made for UI while Paper2D suppose to do 2D gameplay (but i didn’t hear any development on that). Read how to shrink build of UE4 in normal means there defiantly tutorials, thats your best bet
There way to make Slate only applications (UMG is powered by Slate) but considering Blueprint system sits Engine module and whole point of UMG is make Slate usable in blueprints, it makes no sense to even try that as you could as well build it normally, i also not sure about legality of to EULA, if you can distribute such application.