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I suspect /hup/ is unavoidable if you’re going to aspirate the initial consonant. In other contexts (what, why, wheel) /hw/ is distinct, but /hwu/ is nearly impossible to say or hear distinctly. It’s bound to drop back to /hu/ no matter how hard you try to make it clear.
(Edit: I really shouldn’t say aspirate here; it’s not the same thing. But you probably know what I mean.)