Scene transitions -> All volumes are to share the same blendable, the blend distance may possibly allow for a smooth blend also (and if not should be fixed by Epic as it blocks other uses of blendables as well).
Layered Animation and interaction -> You can build a skybox UE1/UE2 style, and then render a real time cubemap from that to have all animations and sky dome interactions you want. The capture camera would need a near clip plane set so it only captures the skybox. Performance impact of that would likely be manageable at such small scale. This would give you the same power and workflow as UE1 and UE2, including all kinds of clouds, moons, etc. type of effects.
PP priority -> You may have a problem with depth of field but most other things should render on top as if the cubemap is a mesh in the world.