Let me say from the start, I am not keen on this idea. It seems a bit clunky and complex. But you asked if there were other possibilities, and this may be possible.

An extra set of transform channels in the hip. Take ytran as an example. We make it so we have this:

actor hip:1
    {
    channels
        {
        translateY ytran
            {
            name ytranHID
            hidden 1
            #etc
            }
        translateY ytran2
            {
            name ytran
            hidden 0
            #etc
            valueOpDeltaAdd
                Figure
                BODY
                HIP_ytran_Kludge
            deltaAddDelta -1.000000
            }
        }
    }

We have a valueParm in the BODY named "HIP_ytran_Kludge", it is slaved to ytran in the hip. Then ytran in the waist is slaved to the valueParm, so it inherits the translation of the hip. Meanwhile back in the hip, ytran2 is also slaved to the valueParm, but with a negative deltaAddDelta, so that it cancels out the pose applied to the hip.

The waist has now adopted the pose applied the the hip, and the hip has not changed. If the user wants to ytran the hip they use ytran2 (display name "ytran").

Poses made from other figures, and those used as walk blends in the WD will be posing the waist, when they "think" they are posing the hip. Transforms applied manually get applied to ytran2 (and yrot2, etc), and pose the hip as intended, without affecting the waist. Poses saved from this figure will save data from both sets of transform channels, and so restore what ever state the figure was in when the pose was saved.

Forign poses and the Walk Designer will be unaware of the extra channels and slaving, and apply the data to the hip. Antonia will surreptitiously cancel the data in the hip and move the values to her waist.

That's the theory, any rate. I'm not saying that we should do it, or that it would work. But you asked for other possibilities, and this may be possible.