Hi!
I’ve just been playing around to achieve this same effect and I think I got something 
.
As you’ve been saying, Scene Color does not include translucent objects, so a way to solve this is to use a Render Target that captures the scene from the camera perspective to fake a new “render pass” which includes all objects.
Then, instead of sampling the Scene Color, just sample the Render Target texture that emulates what it would be the SceneTexture:PostProcessInput0 containing all the scene color (from opaques and translucent objects).
This solution works with a standard material for meshes/sprites so you can have multiple instances on screen. It is not completely straightforward though, as you have to tweak some things to avoid that the Render Target captures the blur sprites themshelves, etc., but it is pretty much isolated from code/blueprints.
Initially, I wanted to solve this as a global postprocess effect, kind of making a custom g-buffer with screen space motion vectors.
Think of it as if the Render Target just renders the sprites used for the radial blur and masks everything else, then in these sprites you draw the direction of the radial blur for each pixel (similar to a normal map when you make a distortion). The postprocess material then samples this “pass” and makes a global blur using the information of the direction from the sampled pixels.
This is how motion vectors work and what is used to make the standard motion blur. You can look at the ‘Buffer Visualization: Velocity’ in the Viewport to see how it looks when fast movements happen.
I didn’t follow this second approach because you need a way to occlude these objects from the main pass but have this info rendered somehow in the g-buffers. You could use a Render Target the other way around, just capturing these blur objects, but you would need to make some Blueprint work to spawn your Niagara Systems and these objects at the same time.
I just kept with the other approach because I don’t really need to optimize this in any way and it’s a little bit more straightforward, but here you have these ideas for anyone who wants to try!