I’v had a similar problem, the solution for our project was to modify engine slightly (just the PostProcessing.cpp file) and reorder separate translucency to be composed AFTER motion blur & TAA pass, this way we have layer that is unaffected by those effects and could display clear UI/HUD with alpha blending on it (all translucent materials marked as ‘after DOF’ are rendered into this layer).
This is not ideal (since particles not affected by DOF are also not anti-aliased), but works quite well, and antialiasing for alphablended objects can be done by hand anyway.