I read that you can create a new material and check “Disable Depth Test” in the Translucency panel. Every mesh with this material should be rendered on top, but I havn’t tried it so far.
And if you want this feature just for the editor, you can try setting bUseEditorCompositing of your PrimitiveComponent to true. I think this is the way they draw the handels in the editor. But again, I’ve never tried it 